"I saw Kenneth Branaugh (not an ancient British surname)"

Actually I think it is. Kenneth Branagh is from Northern Ireland. Of the Protestant persuasion and I imagine regards himself as British as well as Irish. Branagh is claimed to derive from 'Breathnach' which translates literally as Welshman. The progenitor seemingly being a Philip the Welshman who arrived in Ireland with Strongbow in the 12thC. You don't get many names much more ancient than that.