Just had a look on the CAA Web site and Schedule 5 of the ANO seems to be the chapter and verse on this. There is an issue of GASIL from 2001
here which gives a good summary on the front cover of the situation as I understand it, which goes as follows:
Non-FM immune VOR and ILS receivers must be identified with a suitably worded placard and such receivers must not be used to comply with a requirement for the mandatory carriage of radio equipment. In the United Kingdom these requirements are set out in Article 15 and Schedule 5 of the Air Navigation Order (ANO). (Commercial air transport operators with JAR-OPS Air Operators Certificates should refer also to the requirements contained in JAR-OPS.). In addition, national aviation authorities in other countries are entitled to designate airspace where approved radio equipment must be carried.
For flight under instrument flight rules (IFR) in controlled airspace the legal requirements include one VOR receiver (which must be FM immune).To conduct an ILS approach inside or outside controlled airspace, one (FM immune) ILS receiver is required. The reference in Schedule 5 to duplicated VOR equipment relates to public transport IFR flights, whether inside or outside controlled airspace.
Note:This summary does not include all the radio and radio navigation equipment required by the ANO –for which the Order itself and Schedule 5 must be relied upon.
It should also be noted that there is a separate issue here - since the introduction of EASA on 28 Sep 03, they are now the regulating authority and the aircraft equipment approval process has been changed. There is a recommendation to contact the SRG at CAA.