I don't see this scenario as being any different regardless of the type of unit (ATC, FIS, A/G). With regards to the passing of an instruction to land, even at an ATC unit a pilot could simply state that they were diverting elsewhere.
There isn't any specific guidance for any type of unit for this scenario in the respective CAP publications. The closest is in MATS Part 1 for ATCOs withholding Take Off Clearances
Withholding Clearance
8.1 The Aerodrome Operator and certain other persons are empowered to prohibit flight and they may instruct controllers to withhold a clearance. A list of the personnel authorised under civil aviation legislation and the procedures to be adopted when detaining aircraft appear in unit instructions.
8.2 If a controller is instructed to withhold take-off clearance, he should take reasonable steps to establish the authenticity and powers of the person giving the instruction.
8.3 In addition a controller shall withhold clearance to take-off when it is known that an aircraft has been detained by a police or HM Customs officer.
8.4 If a controller has not been instructed to withhold clearance but he has reason to believe that a planned flight is liable to endanger life or involve a breach of legislation,
he is to:
a) warn the pilot of the hazardous condition or apparent infringement and obtain an acknowledgement of the message;
b) in the case of an infringement of legislation, warn the pilot that if he does take-off the facts will be reported to the appropriate authority;
c) if the pilot still requests take-off clearance after acknowledging the warning he should be advised, when traffic permits, that there are no traffic reasons to restrict take-off;
d) record the warning and any comment made by the pilot in the ATC Watch Log.
8.5 Phraseology
Because of possible legal action when pilots disregard the warnings described above,or when aircraft which have been detained depart without clearance, it is essentialthat clear and precise messages are passed to the pilots concerned andacknowledgements obtained. The recommended phraseology appears in Appendix E.
Further transmission may be necessary to ascertain the intentions of the pilot.
I imagine therefore that a ATCO/ FISO/ A/G Operator faced with this scenario would have to think on their feet and say something like
"G-AA, message from the Metropolitan Police, you are to land at xyz airfield", and then to confirm the pilot's intentions.