a) popular course
b) over 20% of A level grades are As nowadays
c) you are not allowed to reapply with a deferred place anyway
d) Durham probably won't let you defer (they didn't let me - I had to reapply)
Basically, don't reapply *just* for Oxbridge. I genuinely wanted a gap year, and Durham wouldn't let me defer, so I did. I would have been so gutted if I'd been rejected by Durham the second time round. Luckily I wasn't, but to be honest the admissions system is such a lottery these days that it wouldn't have been too shocking if I had. Chad's is competitive, too, so you did well not to be pooled.
As someone said, you can wait until you get your A level results before withdrawing from university ... I was so unsure about it, due to top-up fees, that I waited until the day before the deadline (beginning of September?) to send UCAS the little slip they send you post-results, saying that I didn't want the place. Durham were probably really pissed off. I'm glad they didn't hold it against me.