Primarily the victory election at the end of WWII where Churchill seemed to assume he'd be back in, in a landslide and he ignored the work being done by Labour memebers amongst the troops. There was a pretty solid army education corps which was (surprise suprise) dominated by left/labour leaning academics, and they put a lot of "why are we fighting" "homes for heroes" "make britain better for you" messages out. Labour got in on a landslide, Clement Atlee was elected to replace Churchill. The Beveridge report [0] was (partially) implemented. This was the birth of the welfare state.

https://en.wikipedia.org/wiki/Beveridge_Report