So all my life I've worn a 34B which I've known for a while isn't the right size - it doesn't fit right and I've also read the bra fitting guru guide so I know you're no longer supposed to add 4 inches to your under bust size!
Finally got around to trying on some in m&s today. Based on my measurements really I should be a 30D but I had major flabby armpit/back bulge in this size, so I asked the fitter for her advice (knowing I would ignore her if she said I was a 34!). She measured me under bust and of course said I was measuring a 34, but looking at me I was more a 32. The 32C didn't fit as well/snugly as the 30D but the flag was slightly less I guess. 32D was just too big all over.
I can't remember my exact measurements, took them a couple of weeks ago and I don't have a tape measure handy now. But I remember I measured the "bra guru" way as a 30D. Under bust I'm definitely 30 inches.
First off, I'd suggest trying somewhere that isn't M&S! Their sizing isn't great - they seem to have been getting skimpier - and all of their cups seem to have the same shape!
The armpit bulge was most likely you needing an extra cup size, or a bra which has wider wires (so the cups aren't as deep but go a bit further round)
I'd maybe try on something like a Freya which are softer, and generally relatively wide (especially their half cup styles) in a 30D and DD and then adjust from there. Some women do need to go up a band size from the measurement if they are very slender/don't have much padding over their ribs but from your description I don't think that's necessarily what's happening here