Do you advertise in Helidata yet?
Avia Press Associates - Helicopter International Magazine - HeliData News & Classified
Otherwise, Rotor Torque (the magazine of the HCGB) would probably get you access to the owners themselves:
Helicopter Club of Great Britain | Rotor Torque
Finally, how about the BHA (British Helicopter Association) handbook?
2013 / 2014 Issue | British Helicopter Association
PS Is the service (that you dare not mention) anything to do with insurance?