Many years ago, I tried an Elila bra that I really loved. At the time, I was a 36FF UK and the direct conversion would have been a 36H since the company uses U.S. sizing. However, the 36F fit best. I'm a bit "fluffier" nowadays, to use a term coined by Gabriel Iglesius, so I knew I was no longer either of those sizes.
In the Elomi Morgan, I'm a 40GG. The direct conversion would be a 40J so I ordered that, a 38J, and a 38I. Why I didn't also order a 40I, I'm not sure.
I ordered the different sizes so that I'd have sister-sizes to accommodate any need for smaller bands or smaller cups.
38K ---> 40J (40GG UK)
38J ---> 40I (40G UK)
38I ---> 40H (40FF UK)
Here is the 40J. You can easily see it does NOT fit. At all.