The Spaceguard Centre is the surprise in Knighton's pocket: a working observatory on the hill above town, concerned with asteroids and comets that come close to Earth. It is small, specialist, and much more memorable than a generic local museum.
Visits are by guided tour, so check times before going. Even if astronomy is only a passing interest, the combination of telescopes, near-Earth-object science, and big border-country views makes it worth the climb.