In the immediate post-World War II years, the UK was riddled with communist spies inside its government and military contractors (many of which were government-run industries after the UK economy became socialized after the war). Among those spies was Britain's Ethel Rosenberg, Melita Norwood.
She died earlier this month. Here's a bit about MI5's failure in the 50s, when it was knee-deep in Soviet moles:
Her security clearance was revoked in 1951 because of concerns that she might be a spy.
But after she was publicly unmasked in 1999 [see here for the book that unmasked her -- TKM] she said idealism, not money, had inspired her.
After she was exposed as a spy, Britain's Security Service MI5 was strongly criticized by the parliamentary committee overseeing the intelligence agencies over the failure to prosecute her.
MI5 completely whiffed on the Kim Philby investigation, expecting it to nab the nice secretary at the Non-Ferrous Metals Research Association is really asking for alot at that point in time.