I've had experience, not in London but in Birmingham, visiting a prison there, where a unit was operated by the local mental health trust. Their experience was that they found it very difficult to deliver the services properly because the mental health treatment unit was located in a new part of the prison that was accessible to people with physical problems. So while it was supposed to be a 34-bed mental health unit, at least half those beds were occupied by people with physical health problems. They weren't really qualified to treat those people, but that was the only place they could get anything, any kind of help with their problems. They just found that the prison culture was not conducive to their being able to deliver their services in the way they needed to, so it wasn't being particularly effective.
Plus, it was a prison for about 1,400, mostly young men. They had 34 beds, about half of which were occupied by people with mental health problems, and that was way, way, way too few for that population. So they had a big problem.