I suspect a declining UK facing a continental hegemonic superpower — anathema to British foreign policy going back half a millennium or more — would not have gone down quietly. And with an overseas empire behind it, the result could well have been an even more ruinous escalation of the Dreadnought Race of the 1900s, possibly leading to a full-blown naval war and/or British attempts to overturn the post-Schlieffen War settlement.

Whatever, it would have been messy — very messy.

(However, the move towards social democracy in Germany was a thing pre-war, and if anything it was the capitulation to nationalism of the moderate left that opened the door to the radical left and communist fringe across Europe; it's interesting to speculate where things might have ended up if the standard bearer for hardline socialism had been someone like Rosa Luxemburg rather than Vladimir Lenin.)