The UK is an interesting case study as for large chunks of its existence the publicly unacknowledged surveillance state, and legally unregulated surveillance state, policed itself on some very hard and fast rules.

MI5 (the internal security service) held very firmly to only gathering intelligence on political movements seeking to overthrow democracy rather than conforming to the wishes of the ruling party at the time. Successive left wing politicians on becoming Prime Minister feared there were large files on them or that the service wouldn't help. Instead they received briefings on the members of their party who were actually members of Militant Tendency, working for Russia or otherwise seeking to undermine democracy in the UK.

Compared to the FBI during the same period they were paragons of virtue. MI5 wasn't, and isn't perfect, but replacing a system of internal morality with outside laws and placement can easily corrupt.