On my first trip using oyster prepay from west hampstead to london
bridge the whole incident was a farce. At london bridge I tried
exiting via the barriers only to be told by a goon that west hampstead
is not a valid PAYG. I was then told by a revenue protection officer
that I had to touch the prepay reader by the pillars on platform 6.
Thus backtracking... Eventually I found the inconspicious prepay
validator in an atrium.

At farringdon I could not find the prepay validators, so I had to exit
the station using my FCC ticket and re enter using my oyster card.
That was a complete mess, considering the FCC and tube platforms are
next to each other. The only thing that I could find was a carnet
validator.

This intergrated transport idea is a complete mess.