Four years ago my son got overall As in all four of his AS subjects but for History, which was the subject he wanted to study at university, he got A in one paper and B on the other so the UMS wasn't stellar. Given that Cambridge expects very high UMS scores he decided to apply to Oxford, which looks harder at GCSE results and the History Aptitude Test.
He got an interview and an AAA offer from Oxford (his eventual results were A* in History and A in three other subjects). He's just graduated with a 2.1 and has had a brilliant time there for all three years.
I should add that UCL gave him an AAA offer as well, and they knew his UMS results. Also worth mentioning that he was at very academic independent school, so no leeway whatsoever on that.
There is nothing whatsoever to lose by applying to Oxford or Cambridge if you have a sensible back up plan.
PS Re the 8 week terms - to add to what was said above, other universities tend to go for a 10/11 week term in the autumn, one of nearly that length in the spring and only a few weeks after Easter principally devoted to doing exams. So in the long run it evens out, and you should also factor in the fact that Oxford and Cambridge undergraduates are getting weekly tutorials/supervisions 1:1 or 2: 1 so the contact time is extremely intense.