My kids (ages 11, 8, and 5) loved England! In London their favorite things were: Tower of London, Princess Diana Memorial Fountain (on a hot day!), in Kensington Gardens the Diana playground, Lego store, M&M store in Leicester Square, playground in St James Park, boat ride on the Thames, Natural History museum, riding the tube, riding in the first row on the top of the double decker buses, souvenir shopping...
They also loved Warwick Castle as someone else mentioned (although we were traveling around the country and stopped there en route from Cotswolds to York). They also loved the city of York (walking the castle walls, York Castle Museum, the Shambles - big Harry Potter fans, watching the street entertainment, and the York Minster had a child's explorer backpack they could borrow with activities, magnifying glass, etc to make things more interesting. We also visited Windsor Castle.
Our first trip to London we rented an air bnb in the Pimlico neighborhood. Very easy to hop on the Victoria line at Pimlico. Also very good bus options nearby.
We are planning a return trip with the grandparents over spring break. We're basing our stay in London with a couple day trips - one day to Canterbury and another day to Hampton Court Palace. Can't wait! Our upcoming trip we're staying at a Premier Inn near the London Eye.