The CAA required all aerodrome authorities to provide their own equipment sometime in the '80s thus allowing ANSPs to be changed without withdrawl of facilities however some companies like NATS Engneering have contracts with the aerodrome to supply and maintain equipment even if there is a change of ANSP like the three you mention.
This didn't stop some ANSPs from 'doing the dirty' with some ATS contracts where (according to one case I heard of) after a change of provider they fired all ATS personnel at end of play on the friday then spent the weekend phoning round to offer new contracts (on reduced terms of course) to about half of the remainding staff to commence on the monday morning.