I really don't know where to begin in trying to answer your questions, and if it weren't for your age I'd probably think that this was a wind-up.
However, I'll try and deal with some of the points, giving you an allowance for your - inevitable given your age - naivety.
To fly commercially you will need a commercial licence. This will require you to pass a class 1 medical. The medical requirements for the forces are generally higher than that required to fly as a civilian pilot, but you still have to meet certain standards.
To obtain a PPL (H) you will require a class 2 medical, but I would suggest that if you have any intention of making a career as a pilot it would be more sensible to take the class 1.
As for your obtaining a PPL, this, assuming you can pass the medical, is down to aptitude, dedication and funding, but this will most definately not allow you to do any form of commercial flying. As for joining the police to fly helicopters, the pilots they use are civilian pilots and are not required to be police constables.
Please don't take offence at my next suggestion, it is meant with the best intentions.
At your age, you would be best concentrating on studying to obtain the best A-level results in appropriate subjects such as maths, physics etc. This is much more valuable to you and is more likely to prepare you for your future career - whatever that might be.
