hmmm this is a tricky one. Most universities ive looked at say that they want you to complete your A levels within 2 years. However, if you were to start completely different subjects you might be okay to take an extra year. Of course, i would check with the universities you plan to apply to first by emailing them.
Alternatively, if you are happy to take an extra year, why not apply for foundation medicine programs? These courses take 6 years to copmplete, rather than 5 and so if you are successful you will graduate as a doctor at the same time as you otherwise would have done. Check that you are eligable first of course if yuou find this to be a more suitable alternative.