I'm sorry, but it's rather silly to dismiss Oxbridge as crap just because your dc didn't get an offer. I don't think that encouraging dc to be bitter is a good way of teaching them to deal with disappointment.
Obviously, if schools are stupid enough to describe kids as a "dead cert", kids are bound to feel bad when it doesn't work out, but surely there are better ways of dealing with it than talking down the place where he originally wanted to go - it doesn't fool anyone, and actually, I think it probably makes it harder for the dc to deal with the rejection.
An old friend's sister studied in Durham. She was very, very happy there and she has done brilliantly since graduating, but I remember going to visit her before making my own university choices and I ruled Durham out because there were so many people with chips on their shoulders about Oxbridge. I thought it was a great city with loads of opportunities, but I felt like I didn't want to be around a bunch of people obsessed with the fact that they weren't somewhere else.
Oxford and Cambridge are great universities, but there are so many other fantastic institutions in the UK. There's just no need to pretend that Cambridge is rubbish, no need to make a big deal of it at all. Just move on, and focus on the exciting future that lies ahead!