You will find the R/T messages for withholding clearances in CAP 413 2.76
CAP 493 Section 1 Chapter 4: Control of Traffic
Withholding Clearance
4.31 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.
4.32 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.
4.33 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.
4.34 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:
1. warn the pilot of the hazardous condition or apparent infringement and obtain an acknowledgement of the message;
2. 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;
3. 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;
4. record the warning and any comment made by the pilot in the ATC Watch Log.