Im not in the cadets. Im old enough for regulars lol. Police is my reserve option, my grandpa was a police sergeant in Camden in London duirng the war and my other grandad an inspector in the met, also helped train special constables. Cadets is always a good thing to put on a CV,Personal statement etc, however don't feel you HAVE to go cadets before joining. A firend of mine (this week actually) was accepted into Bedfordshire police without doing police cadets or special constable. There's no buttering it up, you will get people mocking you or saying things, but it's only on a miniature scale (mainly ACF laughing at your drill), its nothing to worry about.
Do as you wish, try cadets then if you enjoy it volunteer as a special constable, then possibly join regulars when 18+ (if theres a police force left by then). As for me, I'm working on securing a place at the British Foreign and Commonwealth Office after uni in London. Now if you were looking for a political career, I'm the guy to help. Good luck with whatever you do.
*Edit* (I didnt actually answer your question)
Make sure you get a fist aid certificate (I got mine through a course with my school few years ago, CPR on dummies and that jazz)
PE is always good, at the end of the day as a policemen you'll be doing a fair bit or running, regardless of your brains.
Law can help, but really isnt a requirement, most forces only look at uni degrees in law
Most important of all, make sure you do BTEC Level 1 Hair and Beauty
