Adding my own thoughts here, I largely agree with the consensus that Labour will probably get that 20 year run at government Nye Bevan always wanted, but that they'll be left as an isolated (though unoccupied) wreck dealing with reconstruction. There are a few ideas of my own that I'd like to add though;
The Tories are done for as a leading party, and the SS in their current form are done period. The former are probably relegated to being the junior partners of a Liberal Party revived as the only plausible centre-rightist alternative to Labour, while in the place of the latter I've always imagined that an Actionist-successor party along the lines of the otl Italian Social Movement might gain a small but statistically relevant following around the time that movement gained prominence (one might even be tempted to call it the National Front for reasons of convergence with otl).
The radical-left will probably see a rise in some form, both in terms of increased Republican popularity and the rise of any CPGB equivalent. Its doubtful they'd achieve anything big in the short term, but the end of the crown may well be on the table as a concession when Labour is getting desperate for a coalition.
In terms of foreign alignment, I agree with the view that it would mostly be forced to decolonise and withdraw as a Great Power. That period of isolation may come to an end at some point, but the nature of that is essentially up in the air.