May I also recommend Passenger Assistance?
It always worked brilliantly for Paw and me travelling to London for Royal Free appointments etc.
You don’t have that long trek up to and across the concourse (and then down to the taxis) but are met from the train with the “buggy”.
There is also the Passenger Lounge to wait for your buggy to take you down to your train and somebody to help you board , keeping other passengers from jostling you if necessary. It was especially invaluable on one occasion where a late platform change at MK would have meant getting the lift up to the concourse and down to the new platform. Paw could only move slowly with two sticks and no way would we have made it alone, but with Assistance they not only reassured, helped and accompanied us, but phoned over to the platform and held the train. You can imagine the relief from stress especially for me!