Just to ditto what has already been said pretty much...
doesnt matter which you do out of them. but bear in mind the huge cost of this (some places charge £9000 a year currently to grads, but no guarantee this wont change other places are much, much higher). even £9000 is a lot though - as you get no fee loan for your 2nd degree, so would need to find between £40000 and £50000 of your own money, and then living costs etc as well. it is still very competitive getting into vet school as a grad. And more and more people are redoing a levels as well to get in even as a grad - so if its only grades that are holding you back, then is may well be worth redoing alevels now - much cheaper and quicker!!
The way the vet professions is slowly beginning to go in terms of jobs and wages, esp with at least one new vet school in the pipeline, possibly more if rumours are true, then this will push wages down even further. You should make sure that you will be able to find the kind of money it takes to get a second degree, and if this is affordable with the trade off of what wages are/may become. I know vet grads who have had to take nursing jobs for bit before they have found vet jobs - having a massive amount of debt and only earning less than £10,000 a year is not ideal!!