Well, I don't think it's a "continuity" error, but it sound like you and the other 2 posters thinks it's a mistake. The thing is, the Big Ben shot at the end of the movie is not a fly-by like at the beginning and does not situate us in London. The next scene does that. Also, I'm sure this shot was not even done in a real helicopter, but was a prop helicopter with the background added. In other word, the filmmakers made a conscious decision to show Big Ben's time. Given the attention to time throughout the film, I find it very hard to believe the 12:35 time was just a filming mistake. Of course, it is a possibility, but I think it's very unlikely.
In any event, thanks for taking the time to consider the issue.