I’ll second York. We went there for a family holiday at Easter (before my dx) this year (in the snow) and there was plenty to see and do and lots of shopping available, with a big M + S but also lots of different, interesting, individual shops. And a lot of it is pedestrianised so you don’t have to worry about cars. It’s flat too. Would highly recommend the famous Betty’s tea rooms for lunch.
Brighton is lovely, but hilly which may be difficult for your friend with mobility issues.
Manchester is great for shopping, but very spread out, York is much more compact.
After having read all this, I really feel that I want to spend my entire summer holidays in the Uk next year (6 weeks). There are so many lovely places I would like to see.
Or perhaps I should just move there right away, it is honestly my favourite country, I love the language, the way people behave, your hospitality. I have so many pleasant memories from summers spent in England.