Like most "MoD" buildings in London, St Giles was leased. It made the numbers easy to justify the move to ShabbeyWood.
At one time or another I worked on every floor except the basement and ground. Around 1993 the ceiling in my 8th floor office collapsed, making a soggy mess of the engineering drawings for an upgrade.
To the original question, the PE Fleet flew under D/Flying (PE) rules, not the Release to Service. In technical terms, the aircraft usually evolved into the Sample or "Hack", which meant there was no expectation it would ever be fit for Service use again.
The concept still exists, but of course is no longer called the PE Fleet.
Spent many a `time wasting` meeting there in the 80s trying to get `stuff` procured.
We were sat around as well waiting for Main Building to state their requirements.
We usually gave up, stated them ourselves and carried on. In November 1994 I inherited a 5 year old programme. There was one file - containing 5 years of monthly cunning plans, each slipping the programmme a month with the statement "Await Requirement". Wrote the Cardinal Point Spec and had it under contract in 3 days. 4 years later the RN endorsed the requirement retrospectively. CDP then approved funding to enter Development, after the 5th Production set had been delivered and flown. You just gotta know how.