A levels don't just disappear because you do another degree, unfortunately. Many schools still consider them, so your A level grades do still matter.
My understanding is whether to accept someone for graduate entry based on their prior degree is at the discretion of each individual school. In other words, there is no hard and fast rule or answer to your question. The closest you could get IMHO is to contact each vet school's admissions department and ask them directly.
That being said, the graduate entry program tends to 'assume' you have some prior science knowledge, or at least at the schools where it is 4 rather than 5 years it does. So, you are expected to have a working knowledge in some areas, eg cell biology, and this will not be taught again, or might not be taught in as much depth as the non-graduate-entry degree. So I agree that a more general bio degree would be a better choice, from that point of view. Regardless of whether the material is taught or not, it also will prepare you better for the vet med degree, since that is VERY broad.
The flip side of the coin is what if you don't get into vet? What then? If being an audiologist is your plan B, then there is potentially some merit to doing the audiology program. I would again refer you back to the individual universities to see what your options are regarding vet med out of that degree.
Graduate entry is still pretty competitive. Not only do you have all the school leavers that didn't get in but can afford to go the graduate entry route to compete against, you also have the international student pool to compete against. North American students in particular that can afford to do a full-fee vet degree in the UK are not going to be turned off by the fact that it is a graduate degree, because that is NORMAL in the US/Canada. So while some UK students go "another 4 years? I can't wait that long guess I'll move on to plan B"...North American students aren't waiting any longer. And in the case of some American students, potentially aren't paying much more to do their degree in the UK, than they would to go out-of-state in the US. I'm sorry, I don't have any statistics to give you (ie how many accepted out of how many applicants) but if you contact admissions perhaps they can give you those numbers.
Which raises another salient point: just in case you didn't know, doing vet as a second degree means you are paying full fees (9-28,000+ GBP/year, depending on the school) with very little by way of government, academic or bank loans/scholarships/whatever. AKA, that degree comes out of your pocket, or the "bank of mom/dad/family/good friends".
Good luck!