I went to a comp and was completely taught in second hand, 40 year old porta-cabins. 6 people got into oxbridge (3 to each), and the two who didn't get in realistically didn't have a chance (they were predicted b's & c's)
Just go for it if you get mostly A's/couple of B's at AS...I got AABB at AS and was offered AAB for oxford, and it is always worth the go. The accept everyone will be of a certain academic standard, therefore interview with your enthusiasm for the subject is essential.