Allyn, we did not take a tour but we did try to find Harry Potter things in London and in other places. In the Kings Cross train station, the bridge over the tracks where Hagrid first met Harry seems to be gone due to renovation work. We were quite disappointed. Secondly, the King's Cross people built a really tacky looking entrance to Platform 9 3/4. Here's a link to a picture of it.
http://gyazo.com/5829e184aef638b76c526cb7da7585cb
The picture was taken a few years ago. This platform 9 3/4 entrance has been moved to the Western Departure Concourse. Here's a link to a picture of how it looks now.
http://gyazo.com/003452059237601b6bfaa282e6a451b1 For all of the fame Harry Potter has brought to the King's Cross station, they have not been kind to Harry Potter. Nothing close to any similarity to the movie.
If you can get to Lacock, there are places in the Abbey where some Harry Potter filming took place. In the lower levels, there are rooms where Professor Severus Snape conducted class. You can't see any evidence of which rooms it was in but it did take place there.
As a side note, the cauldron in the movies was not a prop. It's real and a couple of hundred years old. It's located in the Lacock Abbey. Here's a link to a picture.
http://gyazo.com/2e98107f0ca11ed49bde2a84f7ae9ef4
Have fun with your Harry Potter adventures. If you are a Da Vinci Code person, check out Temple Church while in London. It's right off of Fleet Street.