Guth - KWC is right. I joined the RN seven weeks after a guy aged 25 (upper age limit of 25 yrs 11 mths for RN aircrew, BTW) who had been, seriously, a London Bus driver. He went back to night school to get the qualifications he should have got at school.
And now he's driving buses again.., as a Captain for BMI Baby.
It can be done, and you do in fact have loads of time. Bear in mind that you need a minimum score of 140 UCAS points - you may be able to do other qualifications (apart from Scottish Highers) to achieve the score, so check with the Careers Office before you commit.