The UK accent

Which one of the dozens if not hundreds of regional and culturally originated dialects and accents do you mean?

actually more modern than that of the US because the US imported the UK one around the time of colonization

That's not how it works.

Like the Spanish- French- and Portuguese-speaking parts of the Americas, American English may have developed from an earlier form of English, but It has since gone through its own parallel evolution, making it just as "modern" as British English.