How can anyone "forget" the station they traveled from? Particularly when said stations are some 20 odd miles apart and presumably look completely different?
I'm sure it's possible if - say - you're tired or stressed or preoccupied with other things.
I don't think I've ever forgotten where I'm coming from, but there's been a couple of occasions (admittedly on short journeys, nothing like Preston-Manchester) when I've forgotten where I'm going
to - to the extent of getting off a train, leaving a station and then thinking, 'Oh bugger, I didn't mean to get out here'. For me that's most likely to happen if I'm doing something slightly different from my normal routine, which tallies with the OP's description of what happened.
Of course that's not really any help to the OP because 'I forgot' is such an easy excuse that you can bet the vast majority of people who claim that are dishonestly trying it on - and since there's no way to tell who the minority who really did forget are, you can well understand a court always rejecting 'I forgot' as an excuse.