I think you'va got a good long term goal set there, but like CO19 says, there are many other options the police as a whole can offer that may well tempt you. For now, if i was you i'd think about how seriously you want to become a regular police officer. It's really not that easy to get in at the moment.
You're going to need a bit more life experience i would have thought, before you're realistically going to pass all the assessments. Chances are you'll also need to spend some time as a PCSO - this is increasingly becoming the route of entry for a lot of officers, then you've actually got to pass all the stages. Competition is high - on my intake there were 4,000 applications for about 50 spaces - an awfull lot of those spaces were filled with people who had police experience i.e. specials, PCSO's, support staff.
Then once you're in it's hard work for the first few months getting your head around everything, and it will require you to change your social habits - no drugs, no bad behaviour, no associating with criminals etc etc.
AS to can a course increase your promotion prospects - no it can't. Anything you do before starting your police training will only help in that you already have the knowledge so should perform better, but it wont be officially recognised - therefore in my opinion the best preparation you can do is to spend a lot of time interacting with people in conflict environments. I did a few years of doorwork before joining and it definately helped me when dealing with people, it is one less thing to master during probation.
All that said - i'm having second thoughts about my career - there's not a lot of excitement in the job at the street level. It really is mainly customer care, form filling, dealing with admin nitemares and "waiting your turn" to do anything remotely fun - i'm seriously considering rejoining the armed forces as RMP, as it has much more to offer in certain areas. . .