Hiya, I used to wear a 38D, until my friend got me a Bravissimo fitting and voucher for my 21st, and I discovered I am actually a 32G! Although quite expensive (upwards of �25 for bra), I still think it's worth paying that for something that fits well and alters my whole shape! Bravissimo don't use a tape measure, they just let you try things on to see what shape fits you best. I was advised to get measured every time I go for a new bra, and also to try on as different brands are entirely different in size and fit.
I do find bra shopping a total chore, and expensive (probably why I was in denial about my size for so long!) but it is worth going a couple of times a year, just so you look great and feel comfy in your underoos.
Apparently the department stores stock good brand name bras-Debenhams do Freya, a make which I have found to be good quality and reasonably priced as far as bigger sizes go.
Good luck pink :) x