Trespass I believe has been done away with in the UK. So safe there.
The only question is whether the idea of saving or preserving lives in the ANO context (aircraft, crew, passengers and any potential casualties resulting from an aircraft malfunction or emergency situation where members of the public and property is at risk) extends to the role of the air ambulance service.
Besides, the aircraft commander can depart from ANO if he feels it is justifiable to do so. Perhaps he would have to defend such actions at a later date but I think and sincerely hope that any sober court in the UK would overthrow any claim against the Air Ambulance acting improperly if it performed its task safely.