Love love love the Ice Rink at the Natural History Museum.
I've been to a few across the Capital (a yearly ritual to get me in the Christmas mood!) and overall I prefer it to Somerset House and Tower of London.
The location is stunning and although it gets busy they don't pack too many people onto the ice. It's also really close to the tube which is a bonus.
I can recommend the hot chocolate too by the way - especially if it was as freezing as it was when I went last year. They have a decent cafe/bar to warm yourself up in.
If you want to go somewhere that screams Christmas, book it!