Bendmeover, that is so sad.
Reassure her that though there will be posh types at Exeter she does not need to mix with them. It will be like school but with a lot more choice. You find your friends and you hang out with them. You recognise other groups exist but if you are not interested in mixing with them and they are not interested in mixing with you, you don’t. She should think about societies she might join where she is likely to meet like minded people.
I doubt Exeter is any ‘posher’ than Durham or Bristol where DD is. And people reinvent themselves. DD went to a ‘posh’ school, and one of her friends went to Exeter and deliberately did not tell anyone where he went to school and made a nice group of down to earth friends and has stayed on in the West Country. DD has a good group of friends from mainly rural areas who like to do outdoorsy things. They were discussing a BNOC (big name on campus) and DD said she knew him. Her friends were surprised as she was taking a different subject and, like them, did not hang out with the loud visible posh group.They were more surprised when she explained she knew him from A level biology.
If she can start thinking positively she should start hearing good things. By many accounts Exeter is a lovely University.
The Gap year thing is difficult. DD had a classic one. Ski season and Camp America, and interning at a cookery school. She learn masses and grew up a lot. The ski season was slave labour but she stuck it out, and is more resilient as a result. But I assume none of this is going to be possible.
Perceived ‘failure’ at this age is so very difficult. Especially as they are high achieving young people, who aren’t failures at all.