I'm pretty sure that as long as the airfield is licenced to accept the type of aircraft in question, then there is no requirement for an ATC service.
I thinkthat the only time the CAA would insist on the provision of an ATC service, is if the airfield operator wishes to publish an official instrument approach procedure.
I know that many smaller airfields have their own NDBs, and ineviatebly some people use them to do let downs.
Also in the past, aircraft operators had "company procedures" at licensed airfields which had ATC but limited approach aids, these procedures utilised a nearby VOR to establish onto the final approach and were approved by the CAA.
Basically, if you want to do an instrument approach to an airfield, ATC must be available.
