I'm honestly shocked noone has said this - Wonderbra.
I have bought a lot of stuff from la senza though, so I'm quite devastated that it's gone. I've bought a few bras from Forever 21, they're veeeery cheap, but I'm usually a cup size up in their shop... They also have this handy promotion on knickers for 5 pairs for €10...
Does anyone know of any good quality brands though? I've bought from Ann summers but honestly can't justify those prices regularly :P