Welsh Air Ambulance=Bond. East Anglia=Sterling (I think). Both AOC holders, both major operators approved by the CAA. You're right, some police units use the helicopter company's PAOC and you are right again, big company = spare aircraft and pilots when either goes bust (if you have the right sort of lease).
Northumberland Air Ambulance is off-line just now due to the aircraft being away for service and no agreement in place for a spare.
I hear that the Sherburn bloke and his mate from Walton Wood have been desperately searching the country for a reputable AOC holder to jump into bed with them and try to convince the CAA that they can cobble together an Air Ambulance operation.