They never got over WWI.
It wasn't so much that they never got over WWI as it was that the time was right for the Communists to infiltrate England's liberal politics.
For centuries the English had had a caste system that discriminated harshly against the lower classes both socially and financially. At war's end the lower classes were eager to adopt socialism because it promised to give them redress for past grievances and elevate their position in return for contributions they made to the war effort.
That was the socialist Labor party's nose under the tent, and Labor's similar strategery after WWII finished the job. Like the frog in the gradually-heated pot of water, the common English got more and more comfortable with a gradually-increasing level of government control, until in time it came to run EVERYTHING.