I'd stick to yeah, Tatillon ~ not because British English speakers don't use yep and yup, but they don't use them everywhere. They do yeah.

Is Yep a British word?

Meaning of yep in English

yes: "Do I press this button?" "Yep, that's right."

Is it okay to say yep?

You are correct that "yeah" and "yep" are informal variants of "yes." In conversation among friends, any form is appropriate, but "yep" has a slightly dismissive tone.