Firstly, you need to think very carefully about your plan to do Forensic Science at University. I attended a talk recently by an academic at the University of Oxford, and he said that a degree in a pure science like Chemistry is much, much better for people intending to do Forensic Science than doing Forensic Science, however counterintuitive it may seem. According to this academic, the biggest recruiters in Forensic Science in the UK do not accept graduates in Forensic Science because they do not think that these courses teach enough pure science for the profession. If you really do want to go into Forensic Science (which, as a warning, is a very saturated area), you would be strongly advised to study something like Chemistry rather than Forensic Science.
Secondly, BTECs are not academic qualifications, whereas Universities are academic institutions. If you find that you do much better at BTECs than A Levels, University might not be the place for you. You probably can get into Forensic Science via BTECs (since Forensic Science degrees seem to have low entry requirements, partly causing the problems detailed above) but I'm not sure if that's a particularly advisable pathway.