It should not be so tight that you can't breathe properly. The problem could be the cup size, the band size, or both.

When you did your loose underbust measurement, which is the one I pay most attention to, were you careful not to dig into the skin and flesh at all? You need to have the tape meaure right up under your breasts, parallel all around, and it should be only just snug enough to stay up, on a neutral breath. It's possible you measured the loose underbust too tightly (although the calculator uses the snug to size the band in most cases, I prefer to use the loose measure). But also, if you compressed your breasts while measuring the bust measurements, that may have led to a too-small cup size. And also, do you have noticeable asymmetry?

Which bras did you order in 32F UK?