There's a brilliant store in Warrington which stocks large cup sizes, even in small back sizes. I've shopped there for years and I can recommend them. Warrington's a really nice shopping centre, with a brand new mall, for a day out.
Hancock & Wood: Lingerie and Nightwear
Bravissimo has a shop in Manchester and I believe one has just opened in Liverpool but, of course, you can shop online with them.
M & S have started stocking larger cup sizes but the range isn't that great.