Pretty sure they have to let you. My sister is technically going into year 14, and this is the 3rd time she is retaking year 12, with a couple different subjects each time. She is at a different school to which she did her first two years though. They have to let her do the whole two years, and they get funded for it, because she is starting the course before she turns 19 years old. The school thoroughly checked the laws on funding and this, as they had never ended up with a year 15 before (plenty resat year 12 but finished when they did A2), but it is completely correct. I don't see why they wouldn't let you when they would still get funded for it.