I was in London yesterday, and happened upon a Bravissimo shop. Haven't been in years as there isn't one locally. I knew they'd say my back size was too big, but suspected cup size would only go up one to compensate for smaller back. Went in in M&S 32DD, Bravissimo fit me with a couple of 30FFs. They fitted really well, all good. But the thing is, they are only ever so slightly bigger in the cups than the M&S bra I was wearing!
I can't help but thinking there's a bit of vanity sizing going on. My boobs are definitely nothing more than medium-sized. There's no way I'd be a 30FF or 32F in any other shop.