Was there a licensed / surveyed FATO area etc available from the point at which you would have liked to depart?
Having a rule that helicopters can only depart from the runway or from the "H" is probably because those departure points have been surveyed for obstacles etc.
Asking to depart from the apron at a licensed aerodrome in your case would be no different to a Cessna 172 saying that "this long straight taxiway will do for our take-off"...........should something go wrong.
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The Air Ambulance is not required to use any aerodrome at all and does not require fire cover in such a case because one seldom finds such services in in farmers field at the side of the Motorway 20 miles from habitation. However, I would expect that the Ops Manual would specify when operations with no fire cover could be conducted.