What if: French invasion of the United Kingdom (1803)

I'd say if Napoleon got over 100,000 men on British soil they'd win. Plus English don't fight guerrilla wars so its wouldn't be like the Peninsular war.

  Teleportation?   This would be the great difficulty.  I think the French invasion for, the Army of England as it was called, totalled ~200,000 men in all but aside from winning control of the Channel they would face problems with getting the forces across the channel simply because if the fragile nature of the barges being used for this purpose.

  The British had about 500,000+ troops available for defence although many were militia units and of course would have to reach SE England but there were also defences in place and the bulk of the regular army in Britain in the region.  As such even if as much as half the invasion for survived the crossing their going to face a tough fight, at least initially.  If the French can get enough of their army and supplies across and win some big victories early it could end the Napoleonic wars pretty early on but Britain isn't going to be the easiest conquest even after that. 

  The British haven't fought defensive guerilla wars for nearly a millennium simply because we're been pretty successful in keeping enemies at a safer distance.  However given the level of national identity and historical distrust of French monarchies I think there would be some prolonged resistance, albeit restricted by the relative lack of suitable terrain.

  I don't know to be honest.  After the suppression of the 1797 rebellion I think it was fairly quiet while many Irish were of course in the British army itself.  Also you have the possibility in the event of a new rebellion to send troops quickly over from the UK.  As such possibly not that big a garrison. 

  Of course if such an uprising occurred during a successful French bid to get a landing it could be difficult for both Britain and the Protestants and loyalists in Ireland.  It might led to large parts of Ireland being lost until the war in SE England with the French was resolved.

  Well they can try, like they did in 1798.  However the relative strengths of the British and French navies have shifted further in Britain's favour.  Also in 1798 the French only sent a small force,  about 1000 men.  Trying to get a sizeable army there, enough to give a chance of taking over Ireland would be a lot more difficult as it needs proper transports for the men and equipment, as well as escorting during the entire mission.  The sort of barges used for the proposed cross-channel invasion would definitely go down over such a voyage and ordinary transports would need escorting.  A much more difficult task. 

  Also after the defeat in 1798 the rebels in Ireland are much weaker so there would probably be little help for any French invasion.

  Few things are utterly impossible but in both cases it needs substantial changes in favour of the attackers to give them a realistic change.  Mind you in both cases that does involve a lot of hindsight and at the time a lot of the concerns in Britain is understandable.

  Not while there was a clear foreign threat I think.  Possibility with different circumstances that the aftermath of a successful [or possibly even more so an unsuccessful] American revolution prompt an uprising in Britain rather than France.

  He claimed but he was no expert on naval warfare, as shown by the killing of his own troops during a demonstration exercise during this period. 

  Given the quality of the transports available then if somehow the French could stop the RN interfering fro that period, the question is how many would get across - rather than going to the bottom - and in what condition.  Also unless the RN is decisively defeated, which seems pretty unlikely, they have to maintain supply lines to get gunpowder, ammo etc as its uncertain how much they could steal from the British.