Its over 20 years since I lived in London, so I might be wrong about this, but I seem to remember that a head stop was where a route partially ended. For instance, I used to live in SE London and the 161 went from Woolwich to Chislehurst or Petts Wood. I think one of the stops in Chislehurst was marked as a head stop to remind the driver that this was where his route terminated (and he had to turn round) if he was not timetabled to go as far as Petts Wood. Its something like that.