Highflyer2007
I think in the first instance your Ops Manual, which forms the basis of your AOC (and you might argue a legal platform of sorts) and should provide the minimum categories required for your aircraft types and appropriate guidance on using aerodromes with reduced RFF capability. It is, in my expereince, not unusual to have a provision in your ops manual for the continued operation to a destination aerodrome with 'one' RFF category lower-provided the aerodrome will accept your aircraft and the commander is informed. Reduced RFF capability may only be due to, for example- reduced manpower in the fire station, or a tender out of service- which may or may not significantly affect the overall RFF capability. CAP168 outlines the aerodrome responsibilites and equipment and manpower requirements as Eric T usefully points out.
I have on more than one occasioned negotiated with an aerodrome to up their category by the use of local fire services, at a cost of course- but it meant they/we could meet our minimum RFF requirements.
Also, it is worth consulting with your insurers on the topic if you are working to establish your company's policy on this matter.