I have a rather similar question. Yesterday my daughter left here en
route to Tottenham Hale and the Stansted Express - all, apparently, went
well and she arrived safely.

I briefly considered accompanying her as far as Tottenham Hale, but I
wondered what Oyster would have thought - and charged me - for a journey
that appeared to start and end at Brixton Underground station, assuming
that I had not gone through the barriers at TH. A ODTC wouldn't have
had this problem, of course....
--
"Mrs Redboots"
http://www.amsmyth.demon.co.uk/