I wouldn't join now and transfer later. Go to college whilst you wait.
Doubtless the OP would benefit on the context of the advice a little more...
Trouble is, from my angle, there's an overbearing in the branch so either way it's not a quick fix for those wishing to become a Diver as a Rating. Three years or more possibly. Direct entry Diver, Aircrewman and Naval Police was only introduced because of manning shortfalls in each branch, Diver certainly isn't in that category now, so the future is at the whim of the branch manager.
I guess it depends on the type of further educational qualifications acquired and the vocational application of the grades attained.
For those undertaking a BTEC L3 Engineering course, the Accelerated Apprentice or the Undergrad Apprentice (UGAS) schemes offer a decent start wage of £31,100 (plus £5k pre tax bounty for UGAS) which isn't a bad back-up but arguably an AET, ET(ME) or ET(WE) could attain the same qualifications whilst earning a wage....if they serve long enough.
One alternative would be to complete sixth form (and maybe gain a degree & circa £50k student debt) before joining on £6k less than an accelerated apprentice as a Warfare Officer with possibly the hope of being streamed MCD Officer (MCDO), but for a 16 year old, it's potentially a long old, costly haul.