Melbourne, Australia, have long done this with their Oyster equivalent (Myki). If you exit a station within 15 minutes of entering it there is no charge (display shows 'Change of mind - No charge').Platform tickets are basically an invitation to fare-dodge. Though I do think same-station exit should be free (not £1) provided it's only after a short period of time during which no realistic journey would be feasible (say 20 minutes), because that would save the faff of fixing things if one changes one's mind due to disruption, and would also allow people to walk through some stations e.g. Earl's Court where this can be convenient without a risk of them sneaking onto a train while doing so.