I guess this question was mainly meant for Jo since she’s the one at Oxford, but I’ll answer anyway! To work as a scientist anywhere, including Oxford University, you have to do A levels in science subjects and then do a degree in science.
If you wanted to go to Oxford to do a science degree, you would (I think) need all A’s at level, in science subjects and, probably, maths. If you wanted to work there doing research, you would need to do well in that degree and go on to get a doctorate. However, you should choose where you spend your time to study on a number of factors other than how prestigious people think the university is, such as the quality of the student lifestyle, how friendly your boss would be (after you met them!) and how nice the city is.
I think that if you want to study for a degree in science at Oxford you have to get really good GCSE and A-level results – over 98% of the students that were admitted to Oxford in 2009 got at least 3 As for their A-levels.
To work at a scientist at the university you don’t necessarily need to have such good exam results. I only got an A and 2 Bs for my A-levels, but because I have a degree and a masters in epidemiology I was allowed to study here for a DPhil (same thing as a PhD) and can work here as a scientist.
There are lots of different science jobs, and some mean you have to have lots of qualifications, but others need a lot less.