Narrowing the services desired might help improve responses. Your open ended question is similar to, "I haven't chosen my RV yet and I need to know which TV I need to order."
Are you looking for canned city bus tours, a specialized activity-focused tour like museums, activity-focused like castle walks or culinary, or cycling, by car or train, etc., and/or one firm covering all the destinations or something else like individualized/small group tours of homes at each location, etc.? You can have separate tours at each city, line up different guides each day, or line up a company that handles everything from lodging to meals, escorts to cultural sites. The options are mind-boggling broad.
Personally, a custom structured multi-day, multi-city tour with Europamundo (https://www.europamundo.com/eng/
) was an amazing provider who covered all aspects of our last trip. My siblings swear by Trafalgar (https://www.trafalgar.com/en-us
) for a one and done provider of canned tours.
I've heard you can't go wrong with this trip: https://www.greatrail.com/au/tours/grand-tour-uk/
Of course, the pandemic may have affected each firm differently.