I believe the requirement for the provision of an ATC service comes from the following taken from the ANO;
In the case of an aerodrome (other than a Government aerodrome) in respect of which there is equipment for providing hold ing aid, let-down aid or approach aid by radio or radar, the person in charge of the aerodrome shall: (a) inform the CAA in advance of the periods during and times at which any such equipment is to be in operation for the purpose of providing such aid as is specified by the said person; and (b) during any period and at such times as are notified, cause an approach control service to be provided.