According to CAP 493 (providing information on Air Traffic Services - found it!) an RCS may be provided to an aircraft operating under IFR, SVFR or VFR. The controller will issue instructions which:
* an IFR flight must comply with
* a VFR/SVFR flight will comply unless informing the controller otherwise
and is provided in controlled airspace only.
All is 'revealed' in CAP 493 sect 1.2-1.5
Fascinating reading
--rob