We have traveled for years using RS tips and guidebooks. Unfortunately I now have limited ability to walk and stand for long periods. Any suggestions are appreciated.
I don't know that tours will be the solution, versus remaining independent and taking your time. In cities, I would trade public transport for taxis taking you door to door. In the countryside, you might want to drive yourselves for more flexibility, although trains do offer boarding assistance (http://www.nationalrail.co.uk/stations_destinations/disabled_passengers.aspx). If stairs are a particular issue, you'd have to discuss them with each hotel or B&B. See also http://www.ricksteves.com/travel-tips/trip-planning/travelers-with-disabilities for tips and links.
I know bike tours exist in London, and London is fairly flat... Maybe you can cycle. I've also seen Segway tours.
There is a Canadian tour company specializing in tours for seniors. It is based in Montreal ... Senior Tours Canada. Their tours include almost everything, including insurance, and are tailored to travellers who need to slow down. Check them out.