It's best to check TfL's Countdown service or download one of the London Bus countdown apps for ioS or Android before setting off.

Last time round in 2012, we had a couple of 197's until mid-afternoon and that was your lot. London Overground will accept London bus users over reasonable routes, (i.e. Honor Oak Park to Surrey Quays which would be undertaken by Route P12 or Forest Hill to Denmark Hill or Peckham Rye changing at Surrey Quays).