I have no idea regarding the CS at oxbridge but I will give advice regarding computer science. Start now if you havent already, use code academy to learn a language, build an application using that language and share it on github, keep building small applications or command line apps and share them publicly. Learn Python or Javascript or both. Then try out C (Learn C the hard way is a good resource although now a purchase of the book is needed) This is important in this field, open source contributions are looked upon fondly and are major factors in how an employer looks at you. It would do you no harm in your application to uni as well if you had a active github account. Best of luck in getting in, and great GCSE results btw!