you've certainly got the grades, but you are right you must get some work experience. 3 weeks is probably fine as long as you spend time in a range of wards/departments like neuro (stroke, parkinsons), outpatients, elderly wards, respiratory (ITU, surgical wards etc), amputee's, pediatrics etc....the more you see the more you can talk about in your personal statement and the uni's like to see that you have experienced all sorts of areas of physio (not just sports- whatever you do don't say you want to go into that area as so many people get rejected for emphasising one area over another).
It's really all about the personal statement (and grades obviously) so really emphasise what makes you the perfect person to be a physio and being in the armed forces I would imagine you have all the skills to be a great physio (team work, communication, decision making etc). To give yourself a better chance though I guess you could do some sort of health related course or you could actually work as a health care assistant or something like that in a hospital- that would also look good on your application.
It took me 2 attempts to get into physio (I did another degree and got a bit of work experience) but I made it even with pretty poor A-levels (compared to yours anyway!), uni's like to see determination and a drive to do the job so if you are committed to it I'm sure you'll get in no problem!
I hope it works out!