The filmmakers were given advice during post production on whether or not the final version would get their desired certificate.
AFAIK, the BBFC only get to see the finished film, not rushes or rough cuts. They can be given screenplays and storyboards though. The director can meet with them and say what he envisions the final film will look like, and they can advise on what they think the certificate will be if things don't change.
There was an interview with Mathew Vaughn where he claims 'not to have changed a single frame' or words to that effect, but for the life of me I can't find it.
Edit: here it is...
http://www.denofgeek.com/movies/mat...aughn-interview-kingsman-x-men-stardust-2-007
It's interesting that when Psycho was put up for a certificate in 1960, the examiner refused on the basis that he 'saw a nipple'. Hitchcock took the film away, 'made the required cut' and the film was passed, even though, Hitchcock didn't change a thing.