I would guess that it might well be technically possible to sue the UK government over illegal activities at Diego Garcia under both British and EU law. Obviously, any "prisoners" who died there should be subject to an inquest to determine the circumstances and causes of their deaths - but that would presuppose information regarding who is held there and their fates coming to light. It no doubt will....eventually.

I don't know what kind of agreements exist between the US and UK governments regarding DG - they're no doubt highly classified secrets - but the fact of an official UK presence there suggests that we still assert territorial, and therefore, presumably, legal  sovereignty over the island.