I'm neither English nor American, but I do like to thank my partner for sex. I don't think it's a "tradition" (although I could be wrong: like I said, I'm not English), I think it's just a feeling that some people want to express. Personally, I don't think it alludes to prostitution, it's just a feeling of gratitude and satisfaction - your partner makes you feel good, so you thank them for it.

(sometimes though I feel it can be a bit kinky? like thanking your partner for spanking you or something like that)

That being said, I understand that this could be seen differently, depending on the country/culture.
This is very interesting!