Hey, the dream is never over. If you consider applying for Master's, PHD's, etc. Oxbridge is still an option. Just because you don't realize the potential of something happening before, doesn't mean you always have to live with that 'what if?' forever - you could still just as easily continue at a higher level. And the benefit now is that I'm assuming they wouldn't be looking as far as your GCSEs.
You say you are happy right now but I know the question of 'what if this happened' lingers, because it's something that happened to me. The only way to understand this better, is to let it fester & think about it. Do you want to still achieve that goal? Are you able to do it currently? Ask those questions and make sure you are confident in your answers. Figure out if it's a silly thing of the past irking your mind, or still a serious goal you still feel you want and could achieve. It sounds like you've thought of it for a while; and if so I'd honestly consider if it is a goal you want.
Regardless of your decision, you sound like you've gotten far in life. It's good to look back and be proud of things every once in a while, like turning things around at A-Level after your GCSEs - if it wasn't for that you may never had the chance to even consider Oxbridge then.
Just my 2 cents.