My wife and I took our neice to London over Easter a few years ago and I am pretty sure we went to the Tower of London on Easter Sunday as well as had tea at the Dorchester. So I would think museums and other tourist things as well as a lot of restaurants would be open. I think that the Eurostar also wasnt running on Saturday as I recall we had to fly back from Paris the day before Easter because the train wasnt running due to the holiday.