Using only the return portion of a London Terminals<->Calais ticket

[Nicholas43]

The man in seat 61, who knows everything, says that I can't do that.
What stops me
(a) in theory
(b) in practice?
I find it a bit hard to believe that P&O record which ticket numbers they accept at Dover, and don't let you on at Calais with ticket number n unless their computer says that ticket n-1 was used for Dover>Calais.
Or is a worry about this dodge part of the reason for the otherwise mysterious 'control tickets'?

 

[PermitToTravel]

I understand that this is what control tickets are for, but don't have any further details

 

[paul1609]

The man in seat 61, who knows everything, says that I can't do that.
What stops me
(a) in theory
(b) in practice?
I find it a bit hard to believe that P&O record which ticket numbers they accept at Dover, and don't let you on at Calais with ticket number n unless their computer says that ticket n-1 was used for Dover>Calais.
Or is a worry about this dodge part of the reason for the otherwise mysterious 'control tickets'?

Its to do with illegal immigrants In Calais, I won't say more.
Its fair to say that these tickets are not that numerous.