Alright, so there are a few things to clarify here.
First, Oxbridge "prestige" does not help to secure pupillage. The university you go to, in and of itself, matters very little, if at all, in pupillage applications. Many sets are now recruiting university blind, meaning that they don't see the name of your university at all, so it literally doesn't matter to them, and I expect that will become more widespread as time moves on. The reason why Oxbridge students secure proportionally more pupillages is because Oxbridge recruit the most able students and have very high teaching standards, so naturally by the end of their degrees those students are likely to be strong candidates for pupillage. But it doesn't mean that going to Oxbridge is essential for a career at the Bar (it isn't) and it doesn't mean that you can't be a strong candidate if you go to another university (you can).
Second, I expect you're looking at the top end commercial sets in London. Most students who are interested in a commercial practice do. Those sets are ultra competitive and recruit a particular type of candidate with extremely strong academic profiles. But they are a tiny percentage of the Bar as a whole, and even the commercial bar itself is much broader than those sets. Whilst a high proportion of the pupils they recruit from Oxbridge, not all of them are, and all of them have extensive CVs beyond that. But you can also have a very good career as a commercial barrister at other sets in London, or sets in other major cities such as Birmingham or Manchester. It doesn't have to be London, and it doesn't have to be at one of the top sets.
Third, good grades do not make you a good pupillage candidate, but you do need good grades in order to have a realistic chance of securing pupillage. Your predicted A-Level grades are great, but once you move on to university things do get more difficult, and you shouldn't assume that you'll just be able to work your way to high grades in any situation. It's important to study subjects that you're interested in, and to go to a university for reasons other than them being highly ranked. You have five universities that you can apply to. If there is another subjects that you are interested in studying at degree level, by all means apply to study that at Oxbridge or other universities. But don't study it just because you think it'll make it easier for you to get into Oxbridge. That sort of thinking has a much higher chance of being detrimental than you realise. Becoming a barrister is not a tick box exercise, and it's not like taking an exam. You need to gain experience not just to say you've done it, but to genuinely develop your insight and skills, and having a life and interests outside of studying is also really important to make you a more rounded candidate. So do make these decisions in the best interests of your future career, but understand that that doesn't just mean taking any route into Oxbridge. You can't game this process. Make the right decisions for you, both academically and personally.