I'm not sure if it's a UK vs US English thing per se, though there are varying rules with each. Prepositions are often not capitalized in title case, though this comes across more as a compound verb to me.
Anyway, "Log In" does look pretty bad IMO, due to the sans serif font. Even "Sign Up" looks a little off, so we've actually just standardized with "Sign up".
Anyway, "Log In" does look pretty bad IMO, due to the sans serif font. Even "Sign Up" looks a little off, so we've actually just standardized with "Sign up".