Go to University first. It will benefit you massively during training and through your career. It will also provide a back-up for you should the Navy not work out. There is no harm going to your AFCO to enquire about potential sponsorship and it would also be of benefit if, whilst at uni, you joined the local University Royal Navy Unit (URNU).
In terms of the fitness, it's a 2.4km run on a treadmill done at your local Fitness First? gym I believe. The RT test consists of 4 sections, literacy, numeracy, mechanical and comprehension tests.
The AIB is now 1.5 days and tests on the same things as the RT test, but in more detail, a leadership exercise, planning exercise, essay and fitness test. I wouldn't be concerned with that at this stage however.