Thanks Paul, however here's what TFL's own guidelines say about Revenue Protection Inspectors:

'Revenue Protection Inspectors issue either a Penalty Fare or a prosecution notice and clear guidelines are provided for determining which sanction is appropriate. Broadly speaking, penalty fares are issued in cases of suspected passenger misunderstanding or less severe fare evasion. Prosecution notices are issued in cases of suspected intention to evade or more severe fare evasion, e.g., non-payment.'

I also believe that the card in question is always confiscated rather than being returned to the passenger. If the card isn't registered, how do they even prove that it's the one that was used, since they will obviously not be able to produce the original card?