I’m not actually sure about Britain, even though I’ve lived here for 23 years, but in the country I come from there is something called “treatment punishment”, which is basically “you’ll be in treatment until you are well and safe to let out”, which is basically a prison for the criminally insane. Conditions are similar to prison but instead of being under education and having to work, you’re under treatment full time. If not responsive to treatment you’ll sit there forever. The average span is 22 years IIRC but some sit there for the rest of their lives. The mad professor who murdered a woman he had invited onto his home built submarine is in there, for example.