Flying an ILS (or other IAP) at an airfield with something less than full ATC service is not illegal in the UK but can only be done privately, i.e. if the airfield never has full ATC then the procedure will not be published in the AIP but could be flown by private arrangement/approval (this is the case with IAPs at airports in the Scottish islands where there is only AFIS).
However this is all about to change. The CAA consulted last year on making it illegal to fly an unapproved IAP. Here's the CAA's summary of the proposed changes:
At present, only those Instrument Approach Procedures (IAPs) which have an associated Approach Control Service provided by an Air Traffic Control unit may be notified in the UK Aeronautical Information Publication (AIP). Whilst an Approach Control Service will continue to be the norm for control of traffic flying the majority of IAPs, the CAA intends to amend Article 90 of the Air Navigation Order (ANO) and associated Civil Aviation Publications (CAPs) to improve awareness of the presence of IAPs and thus safety. These amendments will allow certain IAPs to be notified where there is an Aerodrome Flight Information Service (AFIS) unit or where the Air Traffic Control unit is providing only an Aerodrome Control Service. Such units will be subject to approval by the CAA. These measures will affect some ATC and AFIS units at aerodromes and also individuals currently using non-notified IAPs. Flying an IAP not approved and notified by the CAA will be prohibited.