If your planning on visiting the National Science Museum, I recommend you spend the whole day there, it's absolutely huge.
This is a good point. The same is true of the National History Museum.
They really deserve a good amount of time dedicated to them, if you really really want to take lots of it in. They have an absolutely vast amount of artefacts and exhibits dedicated to themes all around the world.
You could probably spend a week in there and not see everything they have put out on exhibit.
You've not really said what you're doing in Britain, so, if you're staying for a while, then keep an eye out on the museum.
They change their exhibits every so often in order to get most of it out throughout the year. If they have their Egyptian Exhibits up, they'd be great to see. We sent a lot of pieces back to Egypt (where they were stolen by Egyptians during their riots :/), but there's still a massive collection remaining I believe.
Also, it might not be your cup of tea, but might I recommend that you see if you can get tickets to a WestEnd Musical? It's certainly something you don't do every day. I'd certainly be jealous of you if you went to one.
