Rob you are right, it doesn't have to be the main operators, just anyone who can satisfy the CAA and gain an AOC, Which Virgin HEMS have. The Police units need only gain a PAOC and then employ their pilots/engineers.
Yorkshire Air Ambulance should ask themselves why the established units such as County air ambulance/Devon/Cornwall/Kent/Lincs etc use the AOC of the major companies and don't go to the hassle of trying to gain an AOC of their own?