I had an accident on holiday and needed to request assistance for the return leg. A couple of DMs to BA Holidays on Twitter (as it was then called) and all arranged.

worth letting the crew know you need assistance. You remain on board until your pusher arrives to take you through Heathrow. We had to transfer from wheelchair to buggy, went through the tunnel from B gates on the buggy, then was asked to move to a different buggy to be taken through passport control (through a “buggy” gate), then sat in a holding area waiting for wheelchair transport to the car park.