I would also like to add, Brit Rail is a great way to get around, especially for longer trips, and if you buy a pass in the states before you go it's cheaper than the Brits can get there.  Don't know why it is, but it is.  I've always used the trains, and they're very nice.  You can find out about it by Googling Brit Rail, or you can reach them by searching at Travelocity.com.  Tour buses are great for seeing the usual tourist stuff in London, and they give you the history of things.  You can jump on and off most of them, too.  They have one in Bath, too.  Bath is lovely for walking, and there are some great views from the hills around there (Solsbury Hill is just outside Bath, for instance :)