holly, some M&S bras go up to a G cup, and aren't too un-pretty. I tried on some F and G cup ones the other day, but am a FF at the mo, so nbg!
Debenhams have a lovely range of bigger cup sizes - up to a G cup and they start at £12 for their own brand (called gorgeous) or they also have freya, fantasie, etc if you want to spend a bit more money.
Or there's always Bravissimo if there's one near you.