Re: There were five in the UK..
There were also two non-academic systems: STC (Standard Telephones and Cables) and the RAE (Royal Aircraft Establishment).
The RAE site was the first MLS (Multi-Level Secure) computer system in the UK, as Multics had just been certified to Orange Book B2 level. I constructed the security trials which we conducted for the RAE as part of the acceptance testing.
If I told you any more, I'd have to kill you...