The P4 bus would take you direct from Honor Oak Park to Ladywell (runs about every 10mins or so). I can run to Ladywell in under 10mins as I'm based closer up towards Crofton Park / Blythe Hill Fields way.

The 171 does goes from Holborn and Waterloo. It's just not so great from Shoreditch/Old Street which is where I always seem to be when I'm out later than I should be! I'm hoping the 21 (which goes from Old St and Liverpool St) to Lewisham will start a nightbus route.