However, technically, if you were to alight at a staffed station, you are liable to pay the correct fare for your journey.
May be hard to enforce, even impossible, so you would get away with it usually.
However, you are committing fare evasion and if you got off at a staffed station knowing you hadn't paid correctly- that is deliberately avoiding paying the correct fare, which could be a day in court.
The only way it could be enforced though is plain clothes RPIs sitting in First waiting for you to get off.