I ordered a Panache Jasmine from Simply Be because it was on super sale, and I wanted to give them a try. They don’t give UK cup sizes, but instead give a US-to-UK size conversion chart.
Unfortunately, the size I ordered (32J) corresponded to a 32FF in the mail instead of the 32G that was listed on the website. Customer Service told me that there was some variance between manufacturers, but the size conversions listed were for Panache specifically. I returned the bra and got a refund for the return shipping, but not the original. Ouch. After several exchanges with Customer Service, they refunded me the return shipping again, which was less than the original shipping by two dollars, but said that the original shipping fee was non-refundable.
The takeaway here is that unfortunately the US size conversion doesn’t seem accurate or consistent on Simply Be’s website. Additionally, shipping is nine dollars. So I’m not sure if there’s a way to guarantee sizing of your bra before you order it.