I once had a long chat with the head of a prestige City of London solicitors who acted as the private solicitors for the then British Prime Minister. This guy told me he would never put a serving Metropolitan Police officer on the stand to give evidence in court without someone else who could give corroborating evidence – because he could not be certain the police officer would be telling truth.
I also once knew and chatted with a guy who ran a very high profile UK private detective agency based in London. He employed people from various sources including ex-SAS men but he told me he did not and would not employ ex-policemen because they were not necessarily honest or trustworthy.
A successful South London criminal once told me that dealing with the police is like going to a restaurant – you always have to pay The Bill.