Hello Buddy!
I was in a similar position last year (except I had finished my degree)
Even during your 3rd yr you should have plenty of time to study for AS level bio, especially using letts revise guides and doing loads of practice questions.
I think if youre not bothered about an extra year, just make sure you nail the UKCAT and have a predicted AS grade. Then you can apply for places like newcastle, barts and sheffield which place a heavy emphasis on the UKCAT score for offering interviews.
GAMSAT would be quite good because st georges for example takes on loads of grads. You can use the score for application to ireland as well. It does require very serious preparation but if you get on with it during the summer, it may well pay dividends