Lets see what good ol' CAP168 Licensing of Aerodromes has to say on the matter.
It's in Chapter 8 RFFS and a key paragraph reads
8.18 It is acknowledged that flights may take place that would not normally require
the use of a licensed aerodrome or, the occupancy of the aircraft, due to the
purpose of the flight, is low. Examples include cargo only flights7, training flights,
positioning flights, end of aircraft life flights or business jets where the number
of seats is lower8 than a scheduled aircraft configuration. In such circumstances
it would be reasonable to determine an appropriate level of RFFS provision by
risk assessment through the aerodrome’s SMS.
And for Chevvron
"Remission"
"8. Remission enables aerodromes to provide RFFS facilities to one category below that determined by the size of the largest aeroplane."
"9. Where the number of movements of the aeroplanes in the highest category normally using the aerodrome is less than 700 in the busiest consecutive three months, the level of protection provided shall not be less than one category below the determined category."
"10. Aerodromes currently promulgating RFFS Category 1 or 2 may apply remission on only one category higher than their promulgated category."
If Cranfield were to transition to EASA Reg 2014/139 a similar rule would apply as ICAO is the source of the standards and recommended practices.
I'm not tendering for any work from Cranfield and my a/c reg is G-BMSA