I think that St. Paul's Cathedral is absolutely incredible. I was surprised by one review that said there's "not that much to see inside" ... I spent a full two hours there! Make sure you climb to the dome and see the views of London (for a better price than the London Eye, I might add). This should definitely be near the top of your London to-do list.
If you own or manage St. Paul's Cathedral, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.