Hi!!! So, I know this is late but one of my friends suffered from this!!! She chose single award science and she didnt end up being able to do biology. This one is really difficult and to be honest its gonna make your education quite difficult!!! But if you're really dedicated to becoming a doctor then:
I would seriously, take a year off school if your already finished with your gcses and that you should ask the secondary or sixth form you previously attended if you could privately sit double award science or the seperate sciences and do a mathematics resit in november and really try to get an A or B (that way you only have to sit the units for biology and chemistry, though you could choose physics) - I'd recommend the English examboard AQA for the sciences as they have the most revision resources as they are the largest examboard so self-study will be easier and you wont have to sit any practical units just two normal science papers, but only if your school sits them. Following this I'd reapply to a college like Belfast MET where taking a year out of school isn't that uncommon and you can get the biology and chemistry a-levels you need.
Of course this is going to be extremely difficult socially as you wont be w/ your friends and its going to cost quite a bit of money but if you're extremely dedicated then i cant see why u cant still be a doctor and in your free time you can get work experience in a hospital or volunteer at a hospital to help your chances of getting in, you could also sit the UCAT!