I think in England now one would rarely hear veterinary, even less so veterinarian which is a North American word and almost only ever hear 'vet' and 'vets'. Also if you do hear veterinary is is mostly pronounced 'vet in ree' as folks can't get their mouths round the word. Vets is accepted by Duolingo here now.
The plural of veterinary is formed using a very simple rule for words ending in Y: change the Y in IES. Again why limit to British English? most people in the world are using American English. I'll use what you want so I don't get blocked out but I urge you to update your vocabulary.