The context is obviously French President Nicolas Sarkozy going to jail yesterday. Historically, two French prime ministers have also been jailed: the Socialist (and Jewish) Léon Blum during WWII, and the collaborationist Pierre Laval (who had also served democratically prewar) afterwards. More recently, both President Jacques Chirac and PM François Fillon received suspended jail sentences, but neither actually spent any time in jail AFAIK.
In the UK, I believe the most senior elected politicians to be jailed were two cabinet ministers: Conservative Jonathan Aitken (in 1999, for perjury during a libel case he filed) and Liberal Democrat Chris Huhne (in 2013, for perverting the course of justice in relation to speeding offenses).