Typically what I suggest to most people is aiming for a 7 both in English Language and Maths. When you apply to some slightly quantative Oxbridge courses that dont require maths alevel they tend to ask for 7 (likewise for humanity courses in english). That said, you also want to achieve good grades in the subjects you want to study for alevel. This is because usually a good grade means you enjoy the subject and it would be good for you to carry on + some sixth forms tend to hsve requirements of 5/6/7 to carry on that subject in upper school.
I think its always rumoured that you need perfect 9/8 to apply for oxbridge but thats not the case- just making sure you pass and in your case aiming for a 6 and above in each subject would be a good target. That said, it also brings confidence have some top grades. So maybe aiming for a 8/9 in subjects you want to continue studying would put you in a strong position.
Also, for oxbridge its ALWAYS the predicted alevel grades that matter. You have to meet their requirement (AAA-A*A*A) so dont worry too much about gcse's- focus more on nailing those subjects that you want to study further.