The Natural History Museum baby change is on the ground floor near the pram entrance – to the right as you look at the museum from outside. It’s a pull down one in their main toilet area.
These toilets are pretty swish, and have always been spotlessly clean when I’ve been. They also have fun animal poo facts on the back of each cubicle door! Special shout out as well that you don’t need to get in a lift to get to these – a rare thing for a baby change.