In that case, maybe Debenhams like you said, or John Lewis. IIRC M and S can be a bit random about measuring, as well as the sizes of the bras they sell.<snip> I haven't a clue what proper size I am but I'm not big. I have Bs and Cs, some of which kind of fit, but not really.
I'll have to get some sort of bra Thora! Even my doctor told me off for not having a maternity bra yesterday (although she did compliment the colour).
If your budget was no problem, and you'd be likely to stay the same size for some time, Rigby and Peller. Not just for their staff's general attitude and service, but because they carry a wide range of the higher quality brands, even in non-average sizes.