As Long as you get a B in maths and English language, it doesnt matter as long as your A levels meet the university requirements.
I would still try for Oxbridge if you want to but im not sure. To be honest, Cambridge and Oxford are quite different in their requirements.
Oxford:
After you send your UCAS application off, you take some sort of AT exam, physics is PAT and maths is MAT. You have to do these entry exams regardless of whether you get an interview or not. If you do well in the entry exam and do well in the interview, you are likely to get an offer. But the interview is like an exam in itself. Then as long as you pretty much meet the course requirements, you'll get a place.
Cambridge:
After you send off you UCAS application, you will do STEP (entry exam for maths), or have to send off some sort of piece of work for History or English and those type of subjects. Sciences dont require any piece of work. Then you will have an inerview, if you do well in the interview, you get an offer but the interviews are hard like Oxford. However, unlike Oxford, Cambridge want your AS Level exam results to be above 90 ums, I only got two exam results that were 90 ums or above so I didnt get an interview.
To be honest, Cambridge and Oxford have very high drop out and depression rates compared to other universities, so even though your degree will be considered of the highest level, you may hate university. But this depends on whether you like lots of pressure and assessment and can deal with that.
AND Russel Group Unis arent all they're cracked up to be. Surrey, Sussex and Bath appear in the top 10 universities in the UK in some league tables and are not Russel Group, places like Queen Mary's, King's college and UCL weren't anywhere near the top 10 this year and are RG. So if you're deciding on your universities based on its RG status, have a rethink as RG universites are just a list of the first universites that only have a good rep because they've been here for so long. If anything, I'd go to a university that is rapidly developing and improving as you're going to be at the heart of that during your degree.