Sorry but with that kind of GCSE profile you have a VERY slim chance of getting into Oxford, even if you get all A/A* at a-level. Believe it or not, for subjects like Physics/Engineering they place a LOT of emphasis on GCSEs as well as PAT. But more on GCSEs according to an admissions tutor I spoke to.
I don't want to sound rude, but with those sorts of grades, do you really think you can do so well at A-level? I think you are aiming too high academically and will be disappointed.