I have severe mobility problems ...
On HSTs▸ , find the train manager & ask for assistance....
Try that on the 19:30 off Paddington, where the train's called 5 minutes before it leaves, and is so crowded that even an athlete couldn't get to the buffet.
Alternative suggestion:
Print on the back of tickets "If you don't have a reservation, please give up your seat if there's someone more in need of it than you are ..."