Why do you worry about these great GCSE's, ok im no oxbridge admissions tutor but provided you have the grades at A level why would you not consider applying if you meet the offer. You do realise that Gcse's may play a part when giving offers if you have two students who are VERY similar i.e A Level grades achivied. As long as you improve all other aspects of your application you could potentially apply anywhere although... Oxbridge, due to the demand will be far more greater, you should be fine though just don't be blinded by the hype of Russell group and find a uni that is great for your course.