It’s so bad that my fiancée has some bras that say she’s a B cup and others that says she’s a D cup. In order to go bra shopping, you have to actually try them on to find out if they fit.
If I had to try on underwear to see if they fit, I might not bother with underwear at all!
OP is wrong. Bra size is the ONLY women’s sizing that is related to specific measurements. It can still take a while to find a comfortable fit based on shapes, but the sizes are standardized across good brands.
Starting point to find the size: Measure the rib cage right under the bust. If even, that’s the number; if odd, round up. Measure the largest size around the bust. Subtract underbust from bust measurements. 1” = A, 2” = B, 3=C, 4=D.
It gets confusing from there in the US because instead of going alphabetically, the US just adds a D for every inch after 4 until some arbitrary letter then goes back to the alphabet. Using UK sizes just follows the alphabet and so is very simple.