> but probably not hugely different from the treatment of many prisoners

It is, particularly for non-violent prisoners

> maybe prisoners of his nature are routinely interned in Belmarsh.

Its often used in high-profile cases.

> And maybe being in your cell for 23 hours a day is routine

It isn't. 60% of the cells at Belmarsh are multi occupancy and inmates at have access to education, workshops and gyms.

You seem to be casting doubt on a lot of claims, without putting in any effort to challenge your own assertions.