Northern allows passengers to get a permit to travel which you then exchange for a ticket. On pressing the relevant button, you get a message explaining what you are about to do, which contains a sentence along the lines of "By electing to receive a Promise to Pay notice, you state that you are not in possession of a payment card." However, there are perfectly legitimate reasons for wanting to pay by cash despite holding a payment card. So am I liable to a penalty if I board a train with a permit to travel, approach the guard, ask to pay in cash and as I'm getting the cash out of my bag he sees that I have a debit card?
Northern's Promise to Pay Notices have no legal basis in the Railway Byelaws or the National Rail Conditions of Travel. There is no requirement to use them to avoid any kind of penalty. However, if you feel like doing so, I highly doubt that any guard would care if you wanted to use cash to buy a ticket and they happened to see a card. It might well be that there are a number of legitimate reasons for not wanting or not being able to use the card (e.g. having forgotten the PIN, it being blocked due to "suspicious activity", not having enough money in your account etc.). It's a non-issue really.
I have been to several stations now where the ticket machines are hidden away somewhere and not signposted. What if I board the train having not seen the ticket machine and assuming that there isn't one?
In theory, you have committed an offence under Byelaw 18(1) of the Railway Byelaws (assuming the machine caters to your intended method of payment). In practice, the guard is more than likely to just sell you a ticket with the usual discounts, or at worst an undiscounted Anytime (Day) Single. If an RPI comes across you before you obtain a ticket, there is the potential for you to be reported for prosecution or to be issued with a Penalty Fare (notwithstanding the fact that most, if not all, such Penalty Fares issued by Northern are not compliant with the relevant Regulations). It is always a good idea to check the "Stations" section of National Rail Enquiries to have a look at the station facilities available if you are boarding at an unfamiliar station, as there is always a map of the station for you to see where the facilities are (though some of the maps are less than clear!).
Some stations have both a ticket office and a ticket machine. If it would be faster to use the former than the latter, what would be the case if I arrived during the office's opening hours with enough time to use the office without missing my train, but using the machine would cause me to miss my train, and the office was closed for some reason? Could I board without a ticket?
So in other words your question is whether you can buy a ticket onboard despite there being ticketing facilities at your boarding station, because you're running too late to use them. In this case, the answer is quite straightforward - no, you cannot board without a ticket unless the guard has authorised you to do so (which, in most cases, I'd have thought they probably would). Your late-running does not negate the existence of the ticketing facilities and there is no entitlement to assume that it will be possible to make a 'quick buy' at the ticket office (after all, what if there is a queue?). If you do board without a ticket then the same potential outcomes as listed in answer to your second question would apply.
