British Isle Tour

Look at Sage Travel, Road Scholar (filter for easier activity level), consider Rabbies (transportation with driver/guide but you buy own site entries and presumably have some flexibility on how much you want to do at those sites; accommodation can be arranged by Rabbies or yourself, so you make sure arrangements work for your mom.) If price is no object, private tour with Rabbies or other can be set to your own pace. You can also travel independently, making your own lodging, transportation (train, day tours, etc), arrangements at your own pace. What a nice daughter you are! Good Luck!
Oops do you want to do a cruise but on smaller ship? Viking's British Isles cruise included excursions are mostly panoramic bus tours with some easy-moderate walking. Sometimes they offer a "slower pace" group but not always! You could proactively find out where and when the group is supposed to meet for the return bus, and take your own walk; or arrange private excursions. Viking ships are <1000 passengers although the ships are relatively large (like a ~2300 passsenger major cruise line ship) so very spacious. Good she's walking with cane, because I've read (Cruise Critic forums) strollers are difficult/impossible to take on the gangways.

Hopefully this forum's UK residents can also give you some ideas on cruise lines or tours!

Not sure what you would count as a "large" cruise ship. There are a couple of cruise lines, which have ships that I wouldn't class as particularly large, but still would probably have just over 1,000 passengers. They do itineraries round the British Isles. Ambassador Cruise line - www.ambassadorcruiseline.com. And Fred Olsen cruises - www.fredolsencruises.com.

There is also a much smaller line, with smaller ships, having a max of say 120 passengers. Noble Caledonian - www.noble-caledonia.co.uk. They are more expensive.

I have not used either of the 2 larger cruise lines, though did travel before the pandemic with a forerunner of Ambassador Cruises - to the Baltic and to Norway, and I thought the cruises were well done and good value for what we got. I have also done one trip with Noble Caledonian, at the tail end of the pandemic, when they had some stonkingly good prices (not so many people wanted to get back to cruising).. It was a 2 week trip round Scotland and was exceptionally good.

I agree with the recommendation for Road Scholar if you look carefully at the activity level that is shown on each program. There are some at a slower pace but I'm not sure their total Britain one comes with a slower pace. Go to their website www.roadscholar.org

I've done 13 programs with them although I choose ones that are more active than what you want for your Mom but I know the slower paced ones are available. Are you going too? You'd be fine on a RS tour as well and will always find interesting folks on the tours.

Does she have specific places she wants to visit? And British Isles might be too broad a term. I'd not try to include England, Wales, Scotland, Northern Ireland and Ireland in one trip.

Thank you so much for all the suggestions! I really appreciate it. I will be coming along for this trip. I think she is opening up to the idea of maybe leaving out Wales, and Ireland and England are a bit more of a priority than Scotland. She wants to see literary sites, she is a fan of Wordsworth, Beatrix Potter, Bronte sisters, and mentioned a desire to visit the Lake District. Also, family history is mostly Irish so just seeing Irish countryside, and if time permits, Culloden Field in Scotland. I was looking into the RS Tours as well, but worried it might be too strenuous for her, but you have all given me a lot to check out so thank you again!

The Lake District literary experience is something you aren't going to get off a cruise even if it calls at Liverpool or Barrow in Furness.
I don't know what your budget is but Smithsonian Tours do a Lake District tour which seems to match most of what you want except the Brontes.
They take you back to Manchester Airport from whence you can catch a train to Glasgow or Edinburgh, then a Rabbies Tour for Culloden
Rabbies also do a one day tour of Bronte Country, including Haworth, from Manchester.
Then f!y to Ireland and find some kind of tour there.
I don't know what this story is about strollers on the gangway of the shops That may or may not be true of Viking, but it wasn't true of CMV, so is unlikely to be true of Ambassador - their successor. I too was a big fan of CMV. I wish Ambassador were not so downright greedy when it comes to solo cabin pricing, or I would be on them like a shot.

Thank you so much for the information! I like what I have found with Rabbies Tours of the Lake District and I will check out Smithsonian as well. I have sent her some options to look over. I think the main benefit of a cruise would be the ease of leaving luggage on board. She will need to hold on to me for support when walking on uneven terrain. I know we can both travel light and I can manage two small suitcases if it comes to that. Also thinking about the possibility of keeping a base in London and taking day trips. She is pretty open, just regretting she didn't do a walking tour ten years ago. I really want her to have a nice experience. Thanks again for the wonderful suggestions!

Based on your description of her mobility, a Rick Steves tour would be too much for her.

I also would not recommend trying to do the Lake District, Scotland and Ireland in one trip. You could combine the Lake District and Scotland nicely.