The reasons you listed don't sound like you've fully thought it through as to what the job actually entails and are you committed enough to see it through. Please dont take this as personal criticism but 2 close people I know are well one of them was Police officers. My driving instructor was a Police Traffic Officer for 26 years, and I good friend I know has recently moved to being a plain-clothed detective. From talking to them, and doing some research into this a while ago, (Ive always wanted to be a Traffic cop) I found that for the work that is involved the pay is quite low, with both physical and mental drains to deal with day to day. They both have said that on good days its one of the most rewarding jobs, but on the bad, theres very few jobs that can equal it.
So maybe do some more research into it, if your school has a careers library start there.
On to the whole UCAS points from AS/A Levels etc.
Heres a table which shows how many points you get for what.
UCAS Tariff: Tariff tables
And after going through the whole Sixth form thing myself not that long ago, the advice I can give is pick subjects you enjoy/are good at then see what degree courses there are to do with those topics/areas.