Yeah definitely I think you should reapply. Autopool criteria for post Year 13 applicants is three A*s at A2, so I imagine Cambridge feels that getting three A*s is roughly the equivalent to 93% at AS. I'm not sure how it works though if you're doing four A2s; whether they would expect four A*s since you've done four, or whether they only want three, and any extra subjects you've done is just a bonus.
But yeah, I think getting thee A*s will put you in a strong academic position to reapply, possibly more so than people with high UMS at AS, because now you will actually have the grades and not just predictions like the rest of the applicants.
One option you could consider is only taking three A2s this year, as you will have less workload and this may increase the likelihood of getting three A*s. After all I'm sure you'd prefer A*A*A* than A*A*AA (well this is debatable
but still).
Yes there is the important debate about you being out of education for a year, so your science skills will not be as sharp, but I think if you can make your year worth while with a great placement at a hospital or somewhere, then its not that bad. Some people/admissions tutors/whatever, may prefer a medic to take a gap year and get real experience for a year. It's not like you are wanting to apply for mathematics where a year out of studying will be very counterproductive. I believe Queens' college Cambridge are particularly welcoming of gap year applicants anyway, because they like the maturity of applicants once they've had a year out to do something important. If you really want to prove you are still academically able, and have the time, you could take up further maths or additional further maths, to show that your maths skills are not going down the drain during this year out.
Lastly, I think if you hit three A*s, and get good BMAT and UKCAT scores, you will be in a very strong position whatever university it is you're applying to next year. Say you got offers for medicine this year, you could accept these offers, and come summer when you get your results, either you do well enough that you can meet your offers (I imagine none of them will ask for three A*s) and go to uni, or do extremely well and get three A*s, at which point, you can decide to not go to uni this year and reapply. Then I think even if you still don't get an oxbridge offer, the other unis which gave you offers last time, will still be likely to give you an offer this time. From reading TSR recently, it appears that telling UCAS you completely want to withdraw your application to go to uni in 2012, and instead want to reapply next year, is very straight forwards and easy to do.
I just remembered one more thing, when you reapply, you'll be sending off your UCAS application before October 15th. By this time you will want to have planned everything you want to do on your gap year, where you will be working, what you will be doing, and have this written down on your personal statement, so that unis can see if what you are doing is productive. So you'll really only have a month or so after results day to get everything planned for your gap year.