There is much debate over this topic and unfortunately I am away from my computer to get all my references for this issue.

Historians have suggested that during WWII there was a move towards a more left leaning UK, hidden by the fact that by elections in the UK were not properly fought due to a gentleman's agreement between the main parties. Labour had grown in importance during the wartime coalition and began to be seen as a respectable and viable party of government.

Their election manifesto was popular, promising rapid, affordable rebuilding - especially of housing in the urbanized areas that tended to support more left leaning ideas. Promises to nationalize key industry were probably not driving most voters, despite some left wing historians believing that there was a mass turn to the left. Economic growth and development were popular notions at the time.

We must also acknowledge the failing popularity of the Conservative government, especially given some of the comments made by Churchill on the eve of the election. For some, Churchill was part of the old guard and a figure of the past. Comments about a Labour victory being comparable to the Gestapo winning the election probably did little to help his cause.

(For more information and references give me some time to access my records :) )