Most of the chains are wheelchair accessible, as are many larger restaurants anywhere, at least in theory.
Wheelchairs can be bulky and cumbersome animals so avoid poky niches.
Find a place you LIKE, then phone to check how they'd accommodate a wheelchair.
Cutesy Olde Worlde locations don't all need to rebuild to make facilities though many have.
Wheelchairs often place the user's head further back than a normal chair, which can make hearing companions difficult. If possible, go for a quiet place. The hard floors and furnishings made conversation with fellow diners at Wagamama a real strain, though their facilities were otherwise fine.
Make sure your venue knows you'll have a wheelchair user in your party when you book so they can clear all the stored junk from the accessible toilet and its access route. Almost every wheelchair accessible loo is used as a store room for obstructive junk, making it useless/embarrassing for its primary purpose.
ETA You may find this useful
https://yacf.co.uk/forum/index.php?topic=100161.0