Discountinvestigator...what's your reference for saying "use outside published hours is illegal in the UK"?
Article 104 (1) A person shall not cuase or permit and air traffic service equipment to be established or used in the UK otherwise than under and in accordance with an approval granted by the CAA to the person in charge of the equipment.
Article 104 (3) (a) The person in charge of an aeronautical radio station at an aerodrome for which a licence for public use has been granted shall cause to be notified in relation to that aeronautical radio station the type and availability of operation of any service which is available for use by aircraft.
OK, so for aerodromes with licensed for public use with navigation aids located there (almost, but not exclusively all of them. Scatsta, Southend and a few others might not be public use licence but ordinary, in fact Coventry springs to mind as well, but I do not have any sort of official list and do not have time to plod through the AIP trying to work it out) then the hours of service have to be published.
The approved service will include written conditions from the CAA in relation to the provision of full ATC at the time. Therefore, in unwritten terms, or at least invisible to flight crew, you have a requirement to be open and providing ATC for instrument approach procedures to be valid.
Now, if you want to stir up a hornets nest in this area, just ask CAA SRG Flight Operations, Aerodrome and ATC as well as CAA Directorate of Airspace Policy and the corporate legal branch to agree on it. Even if questions were not asked in Westminster, sure as hell they would be asked in Edinburgh at the Scottish Parliament when all the procedures for the outer islands and offshore installations go!