Excellent post JackieS.
Now for my two-pence worth. I would suggest applying for the physio course regardless of being worried about your grades - you may well surprise yourself, and an offer may give you the motivation you need to get the grades they want. Even if you don't get them, many people are being put off applying by the situation in the NHS - there was a thing on the telly a few weeks back where newly qualified physio students were advising against applying! If you aren't that far of the desired grades, they may take you anyway.
You also have the option of being a mature student - once you are over the age of 21 by the time the course starts they take into account life experience. Get plenty of work experience and voluntary work in, not necessarily in physio either, and consider either re-taking your A level exams or other courses, such as those offered by the Open University. I did SK277 Human Biology because I didn't do biology at A level, and I was accepted on that, rather than my Foundation Degree!
I can't offer any opinions on Sports Therapy, but at the moment physio jobs in the NHS are thin on the ground. However you always have the option of working privately, either independently with a sports club, or with higher graded physios in a private clinic, or even for a company like Bupa? You may find that doing the course changes your mind with regards to what you want to do, or solidifies your desire. Either way having a broad base of knowledge is a very good idea, as many subjects that seem to be completely unrelated actually influence each other a lot. If Sports Therapy doesn't give you this broad knowledge, then I would recommend going for physio, as it should make you a better practitioner and far more hireable!