If you definitely want to be a vet then read the below, if it's more generally science that you want to study at uni then generally universities that have better and therefore more competetive science courses will require higher grades than others so it depends on the calibre of university that you would be aiming for.
If you're set on vet school...
If you're taking your science GCSEs this year then I would prioritise them above all your other exams and aim for at least As. As it's pretty near exam time now then if taking them next year is an option then that might be best for you as vet schools have exceptionally strong applicants, both in academic terms and non-academic and most require minimum As in science GCSEs as well as As or Bs in maths and English. I've copied in GCSE requirements from 3 of the UK vet schools below but they're very easy to find online so you can look up the rest for yourself. The general trend is yes you need As in your science GCSEs, although a minority of the very few UK vet schools will not write you off with Bs. You could also consider re-sitting modules that you don't do so well in next year.
RVC:
At least five A grades including:‘Double Science*’ award or "Science and Additional Science"at AA (or an A in Biology and A in Chemistry if taken as separate GCSEs)
And grade A or B in:
English Language
Mathematics
Physics (if taken as a separate GCSE)
Nottingham:
GCSES:
• Minimum of 5 grade As to include Chemistry, Biology and Physics (or science double award).
• Minimum of grade B in Maths and English Language.
• Grade A at AS Level Physics can compensate for achieving grade B at GCSE
Liverpool
A minimum of 10 points at GCSE (where A*/A = 2 points, B = 1 point, C = 0 points) in at least 7 different subjects, to include a minimum of grade B in English, Mathematics and Physics (or dual science including Physics