I'm not enormous by boob standards (40DD) but I find that M&S has the best selection of cute, affordable stuff - they have a couple of ranges that cater for larger cup sizes. Their only downfall for me is that they seem to have overlooked the fact that women with big boobs often have big bums too, lol... and so I find it tricky to always get the matching knickers in my size. It's almost as if the designers don't want to fathom the thought of a larger lady in lovely lingerie!
Evans is also a good spot as they always have larger cup sizes and most of their range is really pretty. Not too expensive either... but if you're lucky enough to have big boobs and a small figure, it's not going to be much use! I do find some of their bras fit really oddly, so I'd recommend trying everything on!