If you are going to consider this seriously, you would need to research things carefully. Many med schools will not accept applicants who have dropped out of a degree course. Nor will most consider applicants who took longer than 2 years to complete the required A levels (usually Chemistry +/- Biology); you would fit into this category if you resit/continue your A levels in science subjects you have already started. There are a few unis that do not mind this scenario (Exeter, BSMS, Lancaster, Liverpool, Keele, UEA, Plymouth, I think off the top of my head), but even some of those will only accept this if they are achieved grades.
So I do not think it is just a simple as deciding you want to do medicine and dropping out. You need to listen carefully to the advice already given, but also check all the university websites and admission policies to see if they exclude people who have failed to complete a degree, or who take longer than 2 years to achieve A levels. If necessary, call them up and ask (but maybe best to give them a few weeks to finish with this year's applicants!). You will then have a list of places is it possible for you to apply. You can also research foundation courses in the same way, as the same rules may well apply to them. Medical school admission rules are tough, and if you are not a standard applicant, they can be even tougher