Unfortunately, GCSE grades are gonna be more important than ever before, thanks to the government's A Level reforms. Whereas before, one would receive AS grades, and then apply to universities with these (alongside predicted grades for the end of A2), now such AS levels no longer exist, and all A Levels are linear (unless you live in Wales or N. Ireland, in which case, you are lucky!! Because y'all still have separate AS and A2).
So now you're going to be applying to universities with your GCSE grades as the only "formal" qualification (as predicted grades are just a guess really). And for a degree course like veterinary science, it is going to be very very competitive! You've gotta remember that you'll be applying for a places against people who have achieved all grade 9s at GCSE plus a slew of extra curricular achievements.
Now that's not meant to scare you, just a heads up, that these GCSEs do matter, and you really need to be aiming for a grade 7 in all your subjects - as the absolute minimum. Ideally all grade 8+