I'll confirm many already listed but give a couple of suggestions. Decide what interests you... history, art, countryside peace and quiet, city hustle and bustle... then pick a smattering from categories to meet those interests. Be sure to take some time to just BE...
York was wonderful, and if you've gone that far north, go on to Durham for the cathedral. AWESOME Norman architecture and the burying place of the Venerable Bede (for history lovers). Oh, and a scene from Harry Potter and the Sorcerer's Stone was filmed in the courtyard...
My daughter loved staying in one of the off the beaten paths towns in the Cotswolds and thoroughly enjoyed walking between villages. She was followed by cows on one path. That is an experience she will NEVER forget... us neither as she made a video.
In the Midlands, where you can tour Chatsworth, there is also Haddon Hall... one of the best preserved old estates as the family left it for untold years while living at another estate so much of the core has not been modernized. It was the filming location for the most recent Jane Eyre movie, and of course Chatsworth stood in for Pemberly in the 2005 Pride and Prejudice. We stayed in Bakewell for that trip.
I'm a history buff so in London the Churchill War Rooms... unbelievable. Museum of London is often overlooked but a gem. Pick what interests you at the British Museum otherwise you could wander for hours, if not days, not really grasping much of anything.
Lunch in the crypt of St Martin in the Fields... right on Trafalgar Square and try to catch a concert too!
I'm taking my niece for her first visit in May... we are planning the Harry Potter Studio Tour... probably not on your mother's list of things to do!