The sizes boux avenue have given are sister sizes ie. They’re practically the same size but the 30f will give u thr best support and if u can’t get tht size u try a 32E. Going down in a cup size and up in the back size is normal it’s just to accommodate ur size but try to go for the 30f as much as possible because the bra will last longer and won’t slack as soon.
Lisa is correct about Marks and Spencer they’re fitting proceedure often sizes people a lot different because it’s so old fashioned. Whenever u buy a bra always try it on anyways becasue some brands are more generous than others.
I got measured in bravissimo post op and had a great time trying various styles and working out what I liked and disliked also how to achieve the best fit fxx