The answer to this question is entirely based on what college you go to. In friendy colleges you'll fit in no matter what because they're...you've guessed it; friendly. In big 'posh' (for want of a better word) ones you possibly won't.
It also depends on which circles you wish to move in. If you're friends with people who have similar backgrounds and interests to you then you'll fit in fine- if you intend to spend your days at Cambridge loitering outside the Pitt Club and trying to shag posh Hawks in Cindies *cough cough* then you possibly won't.
Cambridge seems to me to be a very big boarding school where the pretty girls and sporty guys are popular and those of us who aren't, are not permitted to speak to them or look in their general direction (I'm talking about Trinity here btw, somewhere like Clare is entirely different). People in Cambridge like to reconstruct their old school environments by establishing single sex societies so that they can meet up with single sex societies of the opposite gender and everything will be just like the discos/socials they went to at boarding school. Cambridge is very different to other universities. I find it to be one huge networking social that I try desperately to compete in but without success as I don't have the public school background.
I daresay you'll have a different experience to me, and as I said it depends who you want to fit in with and which college you're at. But by and large being a kid from a crappy comprehensive doesn't make the Cambridge transition overly easy.
Hope that helps and makes some kind of sense.
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