Don't worry about the fact you did a BTEC. I also did a BTEC and now I'm going to York Uni to study Microbiology and Biotechnology! If they accepted you on the course, they obviously think you can do it.
I worry too sometimes about not being good enough, but then i remember its more about how much effort you put in, and achieving D*D*D* takes an awful lot of effort! Plus BTECs have alot of self teaching involved (specially to get the distinctions) which is what it will be like in uni, people who did A levels didn't have that experience, so we may even have a slight advantage to them! Just remember that you are at university to learn, and not just party :P