Just to clarify, FISOs can issue instructions to aircraft, vehicles and personnel on the manoeuvring area (up to the runway holds). In the case of helicopters, once they are in the hover and 'hover taxiing', they can be considered to taxiing like fixed-wing and instructions can be issued.
A/G - clearly not.
If you come across repeated abuse of the rules then COMPLAIN. Not on the freq but in person/by phone. If you don't get anywhere then there is always ATS Standards at Gatwick.