Google 'Longest london bus route' and you'll get the contenders. I always thought it was the 36 from Plumstead to Camden, but it appears this route has been changed since I lived in London 30 odd years ago!
Route 81 is 12 miles long, so will be a good contender. Route 607 was recently extended from Shepherd's Bush to Westfield, making it 13 miles - but that only runs at peak times so probably doesn't count.