The British will hold, a British-Soviet war in the 1980s.

I doubt that any US government is going to leave its nuclear weapons behind,

Overall if we think of the needs of the thread we require:
a) Something to isolate the US from the defence of Europe, at least initially, but having it come in fairly quickly.

b) Something to remove France from play, either having it come under Soviet control or be neutral. I think the latter is marginally more likely, both because it would avoid the question of how the Soviets gained control over a nuclear state and it makes Britain's plight markedly more defensible.

c) Arguably some way for W Germany to be overrun quickly or without fighting so that we have a 'last stand' for Britain and also Britain hasn't lost much of its regular forces and air force seeking to defend Germany.

How about if we have some crisis which results in W Germany accepting full neutrality in return for reunification with the east? This would greatly weaken NATO and possibly cause a de facto collapse with the US withdrawing and France, already slightly removed deciding on a stronger independent nuclear force. Because the reunification involves reserving some influence for the former communist east regime plus the communist cheat like mad and also there is an argument for disarmament for Germany [as obviously there's no longer any east-west tension ] then Germany is largely unarmed and increasingly drawn into the Soviet sphere. The crisis then comes possibly with a staged incident in Germany, prompting claims by the Soviets and their German stooges, of a threatened right wing/fascist coup and a rapid military intervention, then the Soviet forces advancing on into Denmark and the Low Countries. France, alienated from its European neighbours and having received reassurances from Moscow puts its forces on alert and threaten nuclear attack if Soviet forces cross their border but does nothing else and the Soviets leave it alone.

The rump of NATO, probably organised largely around Britain, Italy and the Low Countries as well as Norway and Iceland - with Greece and Turkey's status being unclear, refuse to accept the Soviet actions, aided immediately by Canada and with strong signs that the US will support them quickly - say a forecoming change of President with the President elect already stating he intended to rejoin NATO, decide to fight on. What happens in Italy and Greece-Turkey depends on what Lordroel decides. [Its his thread if I'm not hi jacking it too much]. Ditto with Norway and possibly Sweden/Finland and with Spain. However the most immediate threat would be to Britain as the obvious base for a reforming western alliance to look to rearm and liberate occupied areas. You are likely to have resistance in occupied areas, including much of Germany and possibly unrest in parts of eastern Europe and Russia might also be making a move in the ME which would detach forces and also speed up US intervention.

Anyway a few ideas as to how you might get a ~1980s BoB which isn't totally suicidal for Britain but does give the Soviets a logic for seeking to overrun Britain before aid from N America becomes too strong. It still leaves the question of how they dare risk this in the face of a possible nuclear response from Britain but I don't know what to do about that?

Said that, for me work better if it's London the one that initially go on her own way after NATO collapse, but later change her mind when it's clear that the shit had hit the fan and the red army will soon strike.

Ironically this type of organization can become a reality once the war start