You've pretty much described
Stafford Cripps. He was a key figure in Labour from the 1930s onwards, he flirted with some very radical ideas before the war, such as allowing the government to assume emergency powers on a temporary basis to bring about socialism through means like nationalisation without compensation, as well as being a leading proponent of creating a 'Popular Front' of socialist, liberal, and communists to defeat the national government at the next election. When Labour joined the national government during WW2, he was made Ambassador to the USSR, in part because he was very pro-Soviet, and he somehow ended up taking a lot of the credit for bringing Russia into the war, which meant he became extremely popular and was mooted as a potential alternative to Churchill at one stage of the war. He went onto become Chancellor the Exchequer after the war ended.
There are obstacles to him becoming PM, though, given that he would probably need the endorsement of generally moderate MPs to get him there, and that he can't just succeed to the position as Wallace might have to become POTUS. But it's still doable. I believe that someone started a 'Comrade Cripps' TL on him a while back, I don't know if it was finished.