Hey I'm doing Maths at Oxford and if the Exeter course is anything like ours you'll cover some FP material (just basic use of complex numbers - polar form, De Moivre's theorem, that sort of thing) in the first few weeks but after that the stuff you do will be quite different. Pure maths is essentially split in Algebra and Analysis. Algebra is about studying various algebraic structures, so groups, polynomial rings, vectors etc. and it's most comparable with things like looking for related roots of equations, the factor theorem (that I think you do in C4, FP?).
Analysis isn't much like anything you'll have seen before. It's all about building up theory of the real and complex numbers (starting with sequences and eventually moving on to calculus) starting from very basic axioms and definitions.
In terms of difficulty, university level maths is much harder than A-level, especially for the first few weeks, and it can be a bit of a shock to get your first piece of work back with more red ink than black! Don't worry though you'll soon get used to it and hopefully enjoy being challenged a bit more than you're used to.