Theres also a movie starring Rowan Atkinson as a vicar and his daughter in the movie has sex with her bf in a van. the actress playing his daughter was 17 at the time and performed a topless scene for the movie.
Tamsin Egerton was born on 26/11/1988 but the film Keeping Mum is dated 02/12/2005 so she was actually 18 when it was released. So it is possible that she was 18 when the topless scenes were shot and then they were edited in as the last action before release. Or it could have been a body double (more likely)