I'm really glad that I tried this out! I got a great fit on my first try, so here are some details to help others:
I have a disproportionately large bust which has caused me back pain for years. I usually wear properly fitted UK underwire bras in size 32G. My tight underbust measurement is around 32". My full bust measurement is around 39". I purchased size "34BCD" and I think the fit is just right. It took me about a minute to wriggle in, but now that I'm in the bra, it is comfortable and is not restricting my breathing. I feel very well supported and my back feels good.