Exotic - I wasn't interested in Oxbridge either when I was applying for my undergraduate degree. I went to a school which prided itself on getting people into the top universities, particularly for its scholarship girls who didn't come from traditional 'Oxbridge type' families. I was one of those and I was pushed quite hard to apply to Oxford to do a subject I didn't really want to do beyond A-level. I went to an open day there on a horrible rainy day and I trudged around hating the place. I refused to apply, went to Edinburgh and had a great time. A couple of years later I spent some time in Oxford on a university sports club training camp and fell in love with the place. I loved every minute of my time at Oxford, at Brookes and a year working in the town, but I suspect that I would have hated it as an undergraduate. I also don't think I would have got in which would have been a massive confidence knock at that age.
A lot of it is about applying to the right place at the right time for you and having an open mind about what you want to get out of it. I didn't go into higher education with any intention of being where I am now - it was an evolving process. I am now thinking of a change of direction again - what was right for me in my twenties and early thirties isn't necessarily going to work out for me now, with a second child on the way.
I don't believe that a slip-up at GCSEs, or even A-levels (speaking as someone who didn't work anywhere near hard enough at A-level and could have done better) is the end of any sort of ambition. As long as you have the basic ability and a realistic view of where that ability, and hard work, can get you, there are always opportunities to make changes, or even to go back and start again.
A more recent story than mine - I have a much younger SIL who recently graduated. She is a good student but not great at exams for some reason. She had an offer from Warwick which is top in the UK for her subject, but she dropped a grade and didn't get in. DP and I panicked a little - I think we both reverted to Oxbridge thinking and got in a right flap about her options. She had a place to do the same subject at a small uni in a fairly obscure part of the UK - we did not think this was a good idea and put pressure on her to take a year out and re-sit, perhaps with a stronger CV after a year in relevant work. She stuck to her guns and took the place she had and it turned out to be a very well respected course which looks set to stand her in good stead. Not having learned from this, we were also dubious when she took a job that we considered to be a stop-gap, rather than a career step. A few months on and she has been offered a newly-created role in the company and is doing very well, and has a clear idea of where her next move should be.
There is not only one way of doing things. I hope by the time DS is 18 I will have got that a bit more firmly into my head!