When it comes to GCSEs, "Oxbridge" are not one and the same. You would stand very little chance at Oxford with those grades, because they select based on number and percentage of A* grades, alongside the BMAT. At Cambridge those GCSEs might be slightly below the average applicant's, but if you did well in the BMAT and at AS level (though how they're going to use these for selection given the changes in the system remains to be seen), you might have a shot.