As above, group theory and number theory in FP2 are probably the closest as Interea said. You will probably also be finding eigenvectors & eigenvalues of matrices quite a lot in linear algebra, (often for more marks than A-level! There was an 8 marker on one of my papers for finding normalised eigenvectors for a matrix) though it will be surrounded by more advanced stuff.
Otherwise, proof by contradiction pops up everywhere in maths and gives you the first taste of university level proof. It's not really something you can look at in isolation though, it's something that kind of intertwines work in other areas. Starting in, say, set theory, elementary number theory, real analysis or group theory according to your interests wouldn't be a terrible idea. You can find gentle introductions to most of these.
Talk about maths you like but I wouldn't concentrate too much on the maths you've done in school. You don't want to appear too attached to the school curriculum, it might make it seem like you haven't done much work outside that required for your A-level. Maybe talk about how certain topics at A-level inspired further research or reading: rather than say "I've quite enjoyed matrices", say something like "I quite enjoyed matrices which lead me to read into vector spaces, which gave me a deeper understanding of the calculations I was performing at A-level", etc.