Hi Katie,
Trying to be a vet is a competitive job so you want GCSE's of mainly A* and A with the sciences being most important, mainly because this will help if unis get lots of applications with similar A levels. So your grades aren't anything to worry about. I don't know about BTEC courses but anything extra to add to your CV is good, as lot of getting into university is competing with other applicants and making sure you are noticeable, it probably won't have an effect on your A levels as everything you need will be covered in your courses- everyone says this but don't worry if you find AS levels hard to start with as it is a massive jump from GCSE, it is a lot of work.As for you A levels many of the universities ask for 3 sciences: Biology, Chemistry, physics and Maths all count as sciences at A level. Most places seem to look for 3 A levels and 1 AS level with grades between A*-B obviously with more A/A*s than Bs when picking your subjects it is the sciences which are the most important to get into uni and Russel Group Universities have been known to hold creative subjects such as Art in less regard than other more academic subjects.
Hope this helps and good luck <3