Try buying a ticket from a member on the London Overground staff, they only sell the tickets on the screen even when it's explained to them that the quoted price is wrong, they continue to persist. Very annoying and irritating.

I tried buying a return to Reading via clapham junction, not going into zone 1, it couldn't be done. I have found that prices from London Overground stations are much more expensive than from Southern train stations. It's actualy cheaper to buy a reurn to a southern trainin station like Streatham Hill and then a return from there to Reading.