To the person talking about an Oxbridge conference:
I went to one too, and while yes, 6/7 A*s is the average, that does not mean it is the cut off point. "Average" means many will have had more, and many less.
I went to an awful state school (losing/ mismarking coursework, Leaving us to self teach... the whole works), so I only have 4A*s and the rest As. Obviously, I was and still am extremely worried about this in terms of an Oxbridge application.
I've spoke to several people who are knowledgable or work in admissions. Not one of them has said that I don't stand a chance because my GCSEs aren't good enough. I even went to my sixth forms progression manager to get his opinion, and he thinks my results are worth a shot, especially with extenuating circumstances.
For starters, GCSEs are considered in context, so me coming from a school where to my knowledge I had the best grades in my year will be considered in a much better light than someone with the same grades as me coming from an amazing high performing school. They put a lot of effort into levelling the playing field. Plus, GCSEs are just a small part of the application, AS is just as, if not far more important. And things like admissions tests and PS's also play an important role.
Now I'm not saying below average performance at GCSE doesn't make the application less competitive, of course it does. But it doesn't make it hopeless and unworthy of even being looked at. Please get off your high horse about this, it's making you sound like a real snob, even if you are genuinely a nice person.
To answer OPs question- yes those grades are competitive for Russell group as a whole, but for the more prestigious ones, I see you mentioned Durham- you may wish to contact their admissions team as the standard may be higher. I'm on an SP scheme with Durham, and they requested 5 A*s/ As at GCSE to be eligible, so I'd guess if you can meet this, you'd be in with a shot for Durham. A level grades permitting of course.