Take an adult! The rating 12a means:
12A – Suitable for 12 years and over. No-one younger than 12 may see a ‘12A’ film in a cinema unless accompanied by an adult. No-one younger than 12 may rent or buy a ‘12’ rated video or DVD. Responsibility for allowing under-12s to view lies with the accompanying or supervising adult.
The cinema would be breaking the law if they let you in unaccompanied, and may well ask for ID to prove your age, so the simple solution is to take someone over 18 with you - it does not have to be a parent, a big brother, sister or cousin would be okay.
However, remember that if a film has been rated 12a, it is because the content is not necessarily suitable for under 12s.
Supplement from 09/18/2008 08:45am:
There is another very effective solution which breaks no laws and does not require you to take an adult.