I think this is probably the relevant extract:
Flight Information Service (FIS) provided at an aerodrome is
a service to give information useful for the safe and efficient conduct of flights in the
Aerodrome Traffic Zone. From the information received pilots will be able to decide
the appropriate course of action to be taken to ensure the safety of flight. Generally,
the Flight Information Service Officer is not permitted to issue instructions or advice
to pilots of his own volition. However, in granting or refusing permission under Rule
35 and 36 of the Rules of the Air, FISOs at aerodromes are permitted to pass
instructions to vehicles and personnel operating on the manoeuvring area and
information and instructions to aircraft moving on the apron and specific parts of the
manoeuvring area. Elsewhere on the manoeuvring area and at all times in the air,
information only shall be passed to pilots. Further details on the passing of
instructions by FISOs at aerodromes are contained in CAP 410 Part B Aerodromes