Even though I have my suspension of disbelief dialed to 11, and am overall less critical of the world building than Arrigo (cf. my other thread), the UK massive invasion is the biggest issue I have because, no wonder how you slice it, it can't work on the scale described. The naval construction effort involved (especially if Sweden is also invaded that way) is simply beyond the scope of what the Soviet Union could have achieved, while also developing its land forces, and the distances (and opposition), if starting from the EASTERN Swedish coast (NATO holds the Western Coast) simply too great: remember that Hitler could not invade with overwhelming forces .... and he was starting from Calais. And that does not involve keeping the invasion troops supplied
So, if you need to have Soviet troops in the UK, it needs to be a more limited invasion. I have proposed a possible scenario (SU lavishly finances a "workers' uprising" which mostly exists in the reports of the local KGB agents, who convince the Centrum that the UK is ripe for the taking and that it only needs some elite troops to topple the regime ....before absconding to a sunny locale) It's not the only possible scenario, of course.