I think a lot of veterinary schools won't deliberately favour EPQ students as not every school offers EPQ, so it would be unfair for certain applicants. However, I think you can still use it to your advantage by mentioning it in your personal statment/interviews as something that shows you're interested in the subject. I think that, as long as you don't leave it to the last minute, then you won't find managing your grades and an EPQ at the same time hard (as long as you're doing 3 a-levels). Just pick a topic you're genuinely interested in and it often won't feel like work at all!