You’re close! Both a 34B and a 32D would have the same measurement around their bust, but the bust size alone can’t be used to determine bra size. The rule is to increase band size by 1 increment (which is 2”) and reduce cup size by 1 increment (the letter) to maintain the same volume.
The volume of a 32D is the same as a 34C.
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.