Obvious Mag
Central Defender
The price is the same onboard, however your supposed to have a “Promise to Pay” Ticket (from one of the platform machines) or you can be fined for boarding a train without a Ticket! (don’t think the Conductor can fine you, it has to be by a patrolling Revenue Officer).
You can only use a Promise to Pay with Northern if you have no debit/credit card and there's no ticket office open otherwise you'd be fined.
With our conductors they'll ask you to buy a ticket (if you're honest and pleasant they'll charge you what it would have been pre-boarding and if you're a dickhead to them they'll charge the full fare)
Northern is slightly different as they operate a penalty fare system but again not sure how that would work as it's generally only collected at a Northern ran station and LNER at Newcastle tend to just open the gates on busy days.