This sounds like a quite good foundation for an alternative history game and it seems plausible enough (much more than plot in most games, really) to be able to convince the player and suck him in. Also, if you decide to stick to the 1930s during the actual gameplay, it might stand out, since you don't get many games based around that period. Sure, there are plenty of WWII games (but that's 39-45), Mafia and a few others, but apart from that it's a pretty uncharted time of the XX century. One concern though - does the plot really have to evolve into a Nazi invasion on the US? It sounds dramatic (especially for Americans) and might be popular (Homefront managed to sell a lot of copies, despite negative reviews). but it would also repeat one game already made (Turning Point: Fall of Liberty) and is a bit of a cliche, not to mention that you seem to want to focus the plot on the UK. Also, if you really strive for some degree of historical accuracy/realism, it is much more likely that the Nazis would first invade USSR after defeating Western Europe (that was in fact the original German plan) and then probably take on the Mediterrean basin (which also took place during WW2 - Greece, Northern Africa, etc.), possibly continue to some parts of Asia (shared with their Japanese allies). It is my belief that only after securing their immediate borders would they prepare for intercontinental invasion (having gathered enough resources and possible paths of incursion, as well as securing the Atlantic).

Hopefully that will help. Overall, good idea!

This sounds like a quite good foundation for an alternative history game and it seems plausible enough (much more than plot in most games, really) to be able to convince the player and suck him in. Also, if you decide to stick to the 1930s during the actual gameplay, it might stand out, since you don't get many games based around that period.

There is a reasonably active wargames community oriented around an alt-history setting referred to as VBCW (Very British Civil War), which postulates a fragmenting UK in the late 1930s. It has (for wargamers) some advantages -- many of the uniforms are well known (eg; British Army) and figures easily available. The others can reasonably be "invented" and ported in (eg; partizans, spanish civil war and so on). The alt-historyness means you can set up whatever battles you want, but the outcomes are reasonably well gamed with WW2 era rules -- again most of the vehicles are already modelled and researched from early-WW2, although a lot of civilian vehicles get thrown in.

It's also not *terribly* far fetched that between the abdication crisis and the rise of British fascism prior to WW2 and the Irish already having seceded, that this could have happened.