Good question, but do they still allow you to join at 16?? I think you have to go to the Army Foundation College or Welbeck College at that age. You may be better of going to a college at home, and joining something like the Army Cadets, who will give you an insite into Army life and roles, and you are able to join the TA/OTC at 17.5/18. And, if you join the Army with A levels, or a degree you can then become an officer, meaning that whatever job you do take, infantry or otherwise you will always be using your brain, and have something useful on your cv when you leave.
If it's a job using your brain you want, you probably want to look at the corps ie REME, signals or engineers. You would also gain a few qualifications to set yourself up in civvy street when you leave (in theory at least). Having said that, there are lots of clever people in the teeth arms too so don't discount them. I'd consider CH II's advice too. Why not go to college, rack up a few A-levels and join the TA for a bit? This gives you the option of applying for a commission if you want to. As for good pay, you are looking for a virgin in brothel. Whatever you do, good luck.
Oh, not all infantry are thickies, I have 11 GCSEs, 5 A-levels and I'm doing a Masters degree at a decent uni and I'm TA infantry.
Make sure you tag a good sense of humour on the end of those qualifications...
When you've been stagging on since silly o'clock, your'e pis sed wet through, you've moved location for the 10th time in 24 hrs and you've just watched your mucker take a dump.. you might just need it.
Get yourself in one of the combat support arms such as engineers or signals and get yourself a trade, if you fancy being cold and wet and being out in the field quite a bit go for the engineers good blokes most of the ones i know!! Get yourself a trade out of it, pick something that has good promotion and have a crack at the army pilots course!!