i don't find that the a bra that fits calculator works for me, so don't get hung up on it (people get hung up on the tag size but i find it varies between brands anyway). better to learn what shape of bra works for your body and start there, but its whatever info you look at its always only a starting point (my main thing is finding bras with the cups far enough apart, second the right shape cups)

in m&s i wear a 30C. i generally find one of the big m&s stores should have at least something i can wear and is generally better in store than online, and not just cos i can try them on. the staff there are knowlegeable about their products so they can help you find them when you know what you're after but fairly useless for sizing.