One of the great things about London is that most of the major museums and art galleries are free. Many do have donation boxes and it's good to give what you can because it is lottery and government funded. I remember the days I had to pay to go in, or wait for the free days, so it's a privilege to have the admission fees funded. It counter balances the cost of accommodation in London, which has some of the best museums in the world. I used to live near them and would go in every week for a wander. I would recommend the Victoria and Albert Museum and also the National Gallery. The buildings are spectacular too, and they also serve great tea and coffee.