hi.
I used to work for Rigby and Peller. if you cant get there, and not many can, I would say go for Fantasie bras, Freya,(in Debenhams) Prima donna u can get online.
It was am amazing place to work, truly crazy that so many of us havent aclue what size we are. Fitting in other shops, well i have heard of some places actaully measuring over people's bras and even clothes. Yes some bras are gonna be different sizes in different brands but I would say this.
Buy the smallest back size you feel comfortable in. When this is established ie the number bit, 32,34, only then put that number with a cup size.
If you havent got the bit right around your body you cannot work out the cup. Many many women find they are wearing a bra far to loose round the back/ribs, and end up with it up round their ears, and there fore no support in the cup.
If you have big boobs, say d onwards, and a small back,try and find a bra with thicker straps. Also unles you have smaller boobs, stay away from stretchy material around the band, and on the cups cos it starts to give too easily.
x