Well in year 1 you just do what is prescribed really, but in years 2 and 3 you get a lot of options. Dissertation is in year 3 but I have a rough idea of what I want to do. Try looking at Royal Hollaway uni, not sure what grades it requires but I know it has a classics department which far exceeds the rest of the university- and the offer might not be too high. I'd also recommend reading a few relevant books before you go and maybe including them on your personal statement. I'd probably say try to read the Iliad, Aenead, Suetonius- Life of the Caesars, Hesiod and then maybe a few modern books which give a brief history of Greece and Rome.
EDIT: I've been to Manchester, Newcastle and Leeds unis to see friends and they all seem cool. I have a couple of friends at Leeds who love it and the same at Manchester- can't comment on the course but I can recommend the unis. Newcastle is one of my favourite cities in the UK and they do pretty cheap booze so I'm sure you'd enjoy it there too