It's hard to add to what others have already said, if you are visiting London you just have to do it, and it will be worth it. We pre-booked our tour, including an audio guide, and that all went pretty smoothly, excepting that it was not always easy to find exactly the painting the audio was discussing. Most of the palace is not open to the public, so it doesn't take that long to go through, that surprised me. It's just my opinion, but I think the Palace of Westminster is much more interesting to visit, better value, and a lot less crowded.