Harry Potter isn't coming to London in theme park form, and the Mayor isn't happy:

There's a reason for this, and you suspect it might have something to do with the relative sizes of the countries, the popularity of Florida as a family destination, and the number of people willing to fly to England to see a display case containing one of the 452 pairs of spectacles used in a Harry Potter movie. By the way, we also made a lot of money from another British invention, which were called "the Colonies." Sorry how that all turned out.

England does have a Harry Potter attraction; it's the studio where the movies are made. The article also notes that England has done nothing, theme-park-wise, with the 007 franchise, which would seem a natural. You could even theme it - a 60s area for the Sean Connery films, a tacky wacka-chicka-disco area for the 70s Roger Moore films, a square of the sidewalk devoted to George Lazenby, and so on.