Start with this calculator here.
UK v. US
(Sizing will vary via brand, just as most things tend to do.)
UK sizes are different from US sizes, but the biggest issue is fitting. The US bra industry is a joke. I could go on and on about how inaccurate it is, but I’ll spare us all. Be prepared to have a drastically different size if you use the calculator above. I previously wore a 36C and when I went this route, I jumped to a 32FF (and in reality, I can sister size to a 28GG). Sounds insane, but the fit is a billion times better.
*To avoid confusion, I’ll note that I gained a little in the bust which explains the size jump.
Sister sizes
Also good, if you delve into the world of happy boobies.