It depends, if you can provide proof of your SSC qualifications, and providing Oxbridge accept it, then it should be fine. However, at my school, which is only 6th form, they do make students who haven't taken GCSEs take Maths and English, as I think it may be a requirement for unis. However, I'm pretty sure that most, if not all, Universities will be fine with taking another qualification as long as it is indeed a government certification like you mentioned.