//That election of that seat is declared null and void and a new poll taken with all new candidates.//
Good idea, but the main parties (the ones whose candidates are most likely to win the seat) are unlikely to field any different candidates and even if they did, people who don't like (say) the Tories or Labour are unlikely to vote for anybody else wearing a blue or red rosette. Not that it matters much because (a) the idea (of "None of the Above" as an option) is unlikely to be implemented and (b) even if it was, most of the electorate is so stuck in its ways that "None of the Above" is unlikely to prevail anywhere.