The Prolongation of Parliament Acts of 1940-44 each delayed the election for one year. You can read the debate on the first one here http://hansard.millbanksystems.com/lords/1940/nov/05/prolongation-of-parliament-bill.

This is possible because the UK constitution is unwritten and flexible so Parliament can extend its own life. This only happens in extreme circumstances (it was done in WWI also) because the system would collapse if it were abused. During the war, Britain had a national coalition government containing all the major parties so the extension was passed by all parry agreement.

In the later debates there was an awareness that Commons did not have a very sound mandate but it was still unthinkable to have an election.