Not sure that Fox is guilty of any breach of security, but the only major breach that went unpunished (that I know about) dates from 1982 when former Labour Foreign Affairs minister Edward (Ted) Rowlands MP (now Baron Rowlands), in a debate in Parliament on the Falklands War on 3 April 1982 revealed that we (the British) were reading Argentine diplomatic codes. Rowlands was criticised for revealing this intelligence source, as the likely result of his disclosure was that the Argentinians would secure their systems and the intelligence would dry up. Mrs Thatcher called it "utterly devastating".
As Labour MP for Merthyr Tydfil, Rowlands was an experienced former Foreign Office minister in the Labour government who was involved in earlier Falklands negotiations in the late 1970's
He remained a Labour MP and from 1987 onwards served on the Foreign Affairs Select Committee. His "reading the codes" disclosure obviously had no effect on his future prospects as he was appointed a CBE in 2002, and in June 2004 he was given a life peerage, as Baron Rowlands of Merthyr Tydfil and of Rhymney.
Or the alternate view was that it was sending a message, done it such a way that HMG would proclaim outrage but completely deniable. The agreement done with MP well in advance, thus forcing Argentina to spend time worrying about what secrets were being read and which were not but the real Human Intel info sources still kept under wraps. Classic disinformation technique.
The clear fact that the MP didn't suffer or get abused from press for ever after have always brought me closer to the latter.