LOL. Bras and trousers are sized completely differently, as anyone who knew anything about either would know. Piece of friendly advice - stop digging.
I've worn both, and if you have a 28 or 34 or 40 band, that's how many inches you are on the underbust in the UK. You don't size up 4 or 5 inches unless you want a badly fitting bra, and back/shoulder issues.
Cup goes up with each inch of difference between the under and full bust, roughly speaking. Shape of breast and cut of cup makes a difference.
Been wearing bras since I was 8, might know a thing or two about it unlike you evidently.
Or maybe this is a source you'd prefer
External Link/Members OnlyBased off M&S, and Rigby and Peller fitting guides. Just two of the main places UK women go for proper bra fittings.