Thanks everyone for your replies. 6 weeks on and I'm still unsure about what I should do :/
I've given it a lot of thought and I'm definite that I'd rather study the Exeter course, and would rather live in Exeter. But at the same time I think I would be okay at Cambridge, I like some aspects of the course and the city is alright too, I just don't get the same kind of excitement about it (Which I know is really strange, most people are hugely excited about Oxbridge) I also worry that I don't think I'm really that passionate about my subject, and I want to be able explore other things, and at Cambridge it seems you have to be totally dedicated to your subject!
Does anyone know whether, if I were to firm Cambridge and put Exeter as insurance, I would be able to change my mind later? Say on results day or any time leading up to that? Is there some way of calling up UCAS and asking them to change it etc.?