Hi Ian
Just for your information-ask at security for a wheelchair, they can get a porter to get one for you & help your mother check-in. They will need to know which airline she is flying with as the different handling agents have their own wheelchairs.
Check in will need to see your mother's 'fit to fly' letter, they usually take a copy and give you back the original either at check in or at the gate. Check in will ask if your mother can walk onto the plane (most gates are airbridges but if not they have ambilifts to assist) and assign her a seat accordingly.
The porters will then take your mother through passport control in the wheelchair & to the gate area.
The cabin crew will organise your mother's wheelchair on arrival in the UK.
Hope this helps.
Jo