The proposal is pretty unrealistic. Even if the Soviets had been more successful across Europe and managed to get all the way to the Channel, there would be a clear problem of "well, how do we get our guys across?" The WW2 Soviet Navy was the biggest joke of the Soviet Armed Forces, or at the very least a very unimpressive force. The USN would have a better chance of getting to Britain from Iceland than the Soviets have of getting to it from Germany - and the Soviets would have to fight through Central Europe to get there, first.
However, in the scenario of Britain getting the Post-War Germany treatment, I can see the Irish taking the rest of their island back and thumbing their noses at the beleaguered Brits. The British may complain to the Yankees, but depending on how much the Soviest have in their hands, the remnant would hardly be in any condition to retake Wales back, let alone Ulster.
Plus, with a broken and shattered Britain, many of its colonies will be given their independence guaranteed, with Egypt booting out the Suez garrison and getting a prestige boost to India undergoing civil war after the British were forced to leave to defend the homeland.
If we're talking about a divided Britain (as in, it must happen and cannot just have Britain has a European Post-WW2 Japan) I can see Scotland and perhaps a bit of North England, but the industry, resources, and population will largely remain in Soviet hands. If the US can take the "industrial belt" of Sheffield, Manchester, Liverpool, et al, they'd be in a better position, otherwise the People's Democratic Republic of England/Britain will be the stronger polity.