If you don't stay for the minimum number of days that you have to as an advance notice, I'd say that (if the UK is similar to other EU countries) you'd have to compensate your employer for that. And that would mean something like paying them what they would have paid you for the number of days that you missed, plus any damages resulting from those missing days.

This enforced by law, so if you don't pay, they would just throw you in court.