So Psychology is my back up.... but I know nothing about it

Well you wouldn't get into Cambridge medicine if your AS levels weren't all above 90-95% average (or your best three). Anyway my main point is that apply for psychology in clearing, but i know plenty of people with good a levels which didn't get into medicine after the first year of applying, so don't waste them on psychology courses which won't let you in with a medicine PS. Unless its a psychology course where your comparatively great AS levels will wow them into letting you in, (which means you have to apply to a worse uni), then its not likely that you would get in. Some unis might be more sympathetic though so its worth emailing admissions tutors.

It would be very competitive to become a forensic psychologist - getting on the course is relatively easy, but getting supervision afterwards is more difficult as there simply aren't many jobs in it.

I think you really need to do your research before you commit - you do realise that you can add more choices to your application later? It may be worth doing your medicine application before the deadline (obv) and then adding your other choice(s) when you're more sure of what to do. There's also extra (open before clearing) if you get no offers