this looks like a part of a seperate conversation - which particular set of crimes did you have in mind? colonial wars? the deaths of approx 1.3 million iraqi civilians? the numerous acts of extreme torture? the killing of civilians who were simply traveling to work and bore a vague resemblance to someone the police did not have an accurate picture of?
Three problems - time, money, evidence.
and without lawyers the UK government (which has a lot of lawyers) would simply run rings around us.