British War PlanThis is a more detailed and expanded form of earlier notes, incorporating the mobilisation timetable, force levels, HM Armed Forces deployment and distribution, home defence missions and more.
3 weeks before M-DayIndications of enemy mobilisation detected
Preliminary meetings of Joint Intelligence Committee
Cabinet endorsement of recommendations and passage of inital authorisation through Orders in Council
Notification of Commonwealth Dominions and further endorsement by Committee of Imperial Defence
British alert status raised to BANSHEE State Blue
GovernmentActivation of Regional Seats of Government
Activation of Ministry of Food reserve stocks in readiness for distribution
Preparation of internment camps on Isle of Man
TransportMinistry of Transport and British Railways begins refurbishment of Strategic Steam Reserve
IndustryActivation of Shadow Factories
Royal Ordnance Factories go to 24 hour operations
Civil DefenceROC and Civil Defence Service begin transition to wartime manning
Ministry of Defence12 Military Districts fully activated: London, South East, South West, East, Midlands, North West, North East, Yorkshire, Wales, Lyonesse, Scotland (Highland, Lowland), Ireland
Reserve military bases begin emergency refurbishment and preparation
Military Railways begin operations
British ArmyCall up of specialist reserve units and individual reservists
Activation of British Mobile Force headquarters and support units
The Territorial Army's Imperial Service Force begins deployment, allowing return of regular forces for activation of 24th and 25th Infantry Divisions, reinforcement of the BAOR and other deployments
Progressive reinforcement of regular BAOR units from UK based forces
Royal NavyRN reinforcements flown out to Floating Fortresses
Activation of RNAS Reserve Squadrons and Flights
RN Standby Squadron ordered to active service
Royal Navy Reserve Fleet begins preparations for activation
Reinforcement of at-sea escort squadrons
Reserve ASW Escort Carriers begin preparation
Deployment of 4 additional SSNs to standby stations in the Barents Sea
Royal Air ForceRAF Transport Command begins flights of ammunition and supplies to Middle East Command and Far East Command
Activation of RAuxAF and RAFR transport squadrons
East India Company transport aircraft bought under Crown control
RAF Regiment reserve forces progressively activated to provide additional security to missile bases, strategic air bases and reserve bases
RAF ICBM, MRBM, SAM and ABM squadrons begin full activation preparations
Reinforcement of RAFG to full manning and equipment levels
Royal Space ForceLaunch of oceanic and land surveillance satellites
Police and Domestic CounterintelligenceRoyal Constabulary fully reinforced and progressively deployed
Special Branch and MI5 surveillance of Soviet agents, assets and associates stepped up
SOE units begin deployment to global operating bases
Foreign Intelligence OperationsInfiltration of special operations forces and intelligence operatives into enemy territory via neutral countries
2 weeks before M-DayGovernmentGovernment departments move to 24 hour manning
Bank of England gold reserves moved to safe storage
Home Office issues ‘Emergency Phase One (Warning Stand-by)’ civil protection warning
British alert status raised to BANSHEE State Yellow
TransportMinistry of Shipping begins coordinating shipping movements and prepares convoys
British Railways begins Stage One War Plan
IndustryRoyal Ordnance Factories and Arsenals go to full production
Shadow Factories begin initial operations
CommunicationsBBC begin broadcasts of subtle propaganda films and programming
Gradual activation of reserve broadcasting facilities
Civil DefenceUKWMO and ROC move to wartime preparation status
Crown and Royal FamilyEstablishment of Royal Guard Force
Royal Family begins movement to wartime locations
Ministry of DefenceReturn of serving defence force families and non-essential personnel from abroad
Activation of all colonial regiments and defence forces
British ArmyArmy level headquarters established in Middle East Command and Far East Command
Complete call up of regular Army Reserve and activation of Guards and Light Divisions
Preparatory steps for Home Guard mobilisation
Anti-Aircraft Command and Coastal Artillery units bought to full strength and active service
Deployment of TA units to Special Home Defence Missions: Channel Ports, North Sea Ports, West Coast Ports, East Anglian Air Bases, Strategic Artillery, ICBM bases
Royal Air ForceImperial Reserve Air Fleet (324 Comets, 259 VC-7, 68 Duchesses, 32 Queen, 56 Airships, 24 Skyships) gradually activated
Activation of RAF Home Command and RAF ADGB
RAF shadow squadrons and OCUs activated:
RAF Trainer squadrons to point air defence role
RAF units deployed to Forward Operating Bases
24 hour airborne fighter patrols over Britain initiated
Royal NavyActivation of RN Coastal Forces
University Royal Navy Squadrons called up
Formal activation of Royal Naval Division
Laying of defensive minefields begins
Merchant Navy Reserve Fleet activation begins
Royal Space ForceReinforcement of orbital facilities
Police, Justice and Domestic CounterintelligencePolice leave cancelled and special constables activated
Subversives clandestinely arrested and interned
Small arms and ammunition distributed to territorial Police depots across Britain and Ireland
Execution of condemned criminals expedited
Foreign IntelligenceExtraction of non essential personnel from certain foreign locations
Reinforcement of forward stations
1 week before M-DayGovernmentParliament passes Defence of the Realm Act, Emergency Powers Act, Readiness Act
Parliament prorogued
Government officials moved to wartime locations
Activation of Central Government War Headquarters
Plan PYTHON initiated
Ministry of Food moves reserve stocks to buffer depots
County councils and emergency services implement war plans
Museums and art galleries begin to move valuable exhibits to safe locations in Wales, Scotland and Ireland.
Ministry of HealthOpening of Reserve Hospitals by Ministry of Health
Beginning of clearance of non-essential hospital cases
US and Canadian military hospitals come to full readiness
Ministry of MagicNon-detection spellscreens raised over key facilities
Activation of the Magic Tower Network
IndustryElectricity restrictions phased in for advertising and display purposes
Activation of Strategic Raw Material Stockpile
TransportCombined Imperial Reserve Air Fleet and Imperial Airways move to full military transport duties
Royal Highways dedicated as main military service routes
British Railways begins Stage Two War Plan
Beginning of Essential Service Route closures
Buses and coaches impressed into military transportation service
All merchant shipping formed into convoys escorted by Royal Navy and allied forces
CommunicationsPress censorship by Ministry of Information
Communications System Protection enabled
BBC prepares Wartime Broadcasting System and prepare emergency wartime studio
Public Information films screened by BBC and inserts carried in major newspapers
Household information booklets distributed by Civil Defence Service
Government Telephone Preference Scheme progressively activated
Civil DefenceCivil Defence Service, National Fire Service, United Kingdom Warning and Monitoring Organisation and Royal Observer Corps fully activated
136 CDS Mobile Defence Corps battalions activated
Fire services move to wartime shift patterns of 48 hours on, 24 hours off. Reserve appliance stocks activated
UKWMO test national siren system
Crown and Royal FamilyHMY Britannia to wartime location
Line of succession ensured through global distribution
British ArmyREFORGER: Regular Army round-out divisions and III Corps and IV Corps to Germany
British Mobile Force begins movement to Scandinavia
Reinforcements begin deployment to Iceland
Each Military District assigned a number of Territorial Army Home Service infantry battalions, one or more TA armoured regiments, a TA mounted regiment and assorted artillery and engineer units
Formation of 120 Composite General Reserve Battalions for assignment to Military Districts
Establishment of 240 battalions (7th/8th and 9th/10th Liverpool, Lancaster Fusiliers, Essex, Gloucester, Middlesex, Royal Fusiliers, Manchester, Northumberland, Dwarven, Royal Scots, Black Watch, Royal Irish), forming the Category C/D Army Reserve Mobilisation Divisions
Royal NavyReinforcement of Grand Fleet
Fleets move to wartime operational status and begin deployment
Activation and deployment of Royal Navy Baltic Fleet
Royal Navy Auxiliary Service begins operations
Full scale coastal patrols
Minesweeping around key bases stepped up
Royal Air ForceRAF Bomber Command V-Force to dispersal airfields
Dispersal of RAF Fighter Command reserve squadrons
Constitution of Light Bomber Force from RAFVR
Police, Justice and Domestic CounterintelligenceSelected petrol stations placed under police guard
Armed police guard of key points implemented
Domestic subversive groups banned and membership interned
Release of certain categories of prisoners initiated
M-DayGovernmentBorders closed with Stone of Scone
Rationing enacted by Ministry of Food
Blackout put in place
Restriction on money supply
Closure of the Stock Exchange
Regional Government Headquarters, County Councils Emergency Centres, UKWMO and ROC centres all fully manned 24/7.
Regional Emergency Committees activated
British alert status raised to BANSHEE State Red
CommunicationsFull censorship of press and newspapers in place
BBC begins wartime broadcasting system
TransportEssential Service Routes guarded by Home Guard and Royal Constabulary
Airports and ports closed to public and guarded
Merchant Navy reaches a projected strength of 256,480
Police and Domestic CounterintelligencePolice now operate under full wartime powers
Royal Constabulary: 329,525 (129,000 active and 200,000 reserve)
Ministry of Defence16 and 17 year olds in Army Cadet Corps, Air Training Corps and Sea Cadet Corps called up
126,438 Army Cadets assigned to Home Defence
British ArmyQueen’s Order Two: Full Mobilisation of Territorial Army, Army Reserve, Home Guard and Officer Training Corps
Combined strength of Regular Army, Territorial Army and reserves comes to 4,869,373
1,370,000 additional reservists called up
Activation of Knightly Service Forces
Establishment of 24 Depot Brigades across the 12 Military Districts
Establishment of 24 Training Brigades
Formation of Cavalry Reserve Regiments
British 2nd Army begins activation and preparations for European or global deployment
British 3rd Army and 4th Army begin activation, using Army Reserve and cadre units
Formation of expeditionary corps for potential employment in AFSOUTH
Regular Army: 1,136,429
Regular Reserve: 892,378 (National Service + Class A Reservists + Supplementary Reserve + command and support units for 4 Regular Reserve divisions)
Territorial Army: 1,029,745 (First-line TA divisions, brigades, battalions etc; AA Command; Coastal Artillery; Engineers, Transport, Support units)
Territorial Army Reserve: 1,387,254 (Makes up second-line TA divisions, reinforcement pool, guards)
Army Reserve: 423,567 men from 21-25 (Class A/B Reservists for 4 Army Reserve Divisions, 12 Army Reserve Brigades, 24 Cavalry Reserve Regiments, 24 Training Brigades + Reinforcement Pool)
Home Guard: 2,626,873
Royal NavyQueen's Order Two: RNR and RMR fully activated
25 University Royal Navy Units mobilised and assigned to Coastal Forces
Activation of additional defensive minefields
Royal Navy: 629,528
Royal Marines: 162,254
Royal Naval Reserve: 524,561
Royal Marine Reserves: 87,238
Royal Air ForceQueen's Order 2: RAF Reserve and University Air Squadrons mobilised
Call up of 80 War Emergency Fighter Squadrons using inactive reserve aircraft and manpower pool
Each MD assigned a Regional Air Squadron of training aircraft, helos and civilian planes
RAF calls up 500,000 Class A and B reserves for ground crew, support units, training organisations, repair and maintenance, logistics and supplementary airfield defence
Royal Air Force: 923,487
Royal Auxiliary Air Force: 427,549
RAF Reserve: 682,334
Civil DefenceCivil Defence Corps deploys a total of 1,562,346 personnel
Plus the problem with such a long warm up is that even the early stages would have political and economic costs so governments would probably be reluctant to start it without very good reasons. 
Seriously I would suspect some dispersal and probably Charles or some alternative to somewhere outside the UK, say Canada in case things got really bad.