That's really sad you felt that way, even at the open day. In my experience, at least, a lot of people kept their heads down/weren't hugely social at the interviews, and all the loud obnoxious snobby people didn't end up getting an offer
Were you going around with a general group at the open day or was it the people you'd be studying with if you took up your offer?
It is true that there are fewer people from Yorkshire. I'd like to say two things about that:
1) If you were to turn down your offer, you'd become part of a self-perpetuating cycle whereby people from the North don't see people like them at Oxbridge, so they don't apply or turn down their offers, so there are fewer people from the North, which means people from the North don't see people like them at Oxbridge... if you see what I mean? I'm not saying that you should accept the offer to fly to Yorkshire flag for those who come after you - but I think it's important to stress why there may not be many people from Yorkshire at Oxbridge and how that isn't necessarily entirely Oxbridge's fault...
2) At Oxbridge, like-minded people aren't solely/generally those from your background. The thing that unites (almost) everyone at Oxbridge is a real drive, interest, aptitude and passion for their chosen subject. THAT is where the true like-minded-ness comes from, imho. So yes, there may not be as many people from your background/region as you or Oxford would like - but that doesn't mean you won't find friends or fit in
Personally, I'm a Londoner, so I can't directly empathise with the region/location worries, BUT I was a minority student in various other ways and can appreciate how daunting it may seem. I'm inclined to think your particular fears would be resolved quite soon after starting Oxford, tbh. But that said, if you honestly feel you would hate it and/or would be unhappy there, then there is no shame or harm in turning the Oxford offer down. York is a very good uni overall (I'm not a STEM student, so can't comment on the department or job prospects) and I'm sure it would stand you in good stead for the future