At 18 I was also interested in physics and "genetic biology". I mean, who are cooler than Newton, Einstein and Darwin? It might be best just taking a BSc in genetics, but you can keep your options open by taking a course like the Natural Science Tripos at Cambridge:
http://www.natsci.tripos.cam.ac.uk/subject-information/part-ia
If you do mostly physics at BSc, you could take a conversion course at MSc level, in bioinformatics & genetics, IF YOU ARE STILL FASCINATED BY THE SUBJECT. That there is a such a course indicates that it's a really hot area at the moment, so you might even get a job in it! Check out courses like:
http://www1.imperial.ac.uk/medicine/teaching/postgraduate/moleculargenetics/
Whatever you do at BSc, make sure you choose what you are *really* interested in. Both physics and "genetic biology" are great to take if you eventually want to get a "real job" (programming, finance...), so study either, or both if you remain interested in both.
There are enough options out there to allow you study what you really want to study, just check the promising university sites carefully to see that you get to study what you are really interested in... for instance if you are interested in General Relativity, make sure you can take a course in GR in your final year. If you are interested in mathematical models of evolution & genetics, make sure you can actually take courses in these. Flexible universities should allow you study genetics and GR, if that floats your boat. (It certainly would have floated my boat, but I had to study them in my spare time my University as they didn't enable such flexibility of study!)
In summary, follow your passion, and choose carefully.