Despite what everyone else is saying, in the coming future top universities (Russell Group or not) are going to start looking at GCSE's as they did originally look at AS Levels but with the changing of both GCSE and A-Level systems, they're going to firstly look at your A-Level grades and then GCSE's. I would say A in maths and English, A/A* in the subjects you take for A-Level, then at least a B in everything else or a few C's and you should be good to go for a top of the top university. Yes, applicants to Oxbridge have 27 A*'s at GCSE, but I don't think that really matters if they didn't take the subject at A-Level, if it wasn't a core subject and if the subject wasn't deemed prestigious.