The process is that the wheelchair assistance request is sent by the departing station to LHR who contact Omniserve, the crew onboard will tell the person who meets the flight how many customers require assistance to confirm the numbers match.
If the aircraft is on stand the crew will stay with the customer until they have been handed over.
If the aircraft is off stand then again we confirm with the person meeting the flight the numbers, the coaches take any wheelchair customers to the terminal where in T5 the arrival area is manned by the Omnieserve staff, I am not aware of the arrangement in T3.