That assumes the ticket office was open or at least a ticket machine was working, also isn't there another 'rule' that passengers have to be served with five minutes? If somebody arrives say 10 minutes before their train is due to leave and finds a long queue at the window they can hardly be expected to miss their train can they?
If you have to wait unreasonably longer than 5 minutes at peak times (a notice normally tells you what peak hours are) or 3 minutes off-peak then I'd suggest contacting the operator concerned and try asking them what reasonable endeavours they made to deal with the problem. (E.g drafting in more staff or authorising people to travel and pay at the first reasonable opportunity later on their journey, which could be done by displaying a notice to that effect to cover a staffing shortage.) If they are unable to provide a satisfactory answer and a pattern emerges with it happening on numerous occasions, then you could try taking it up with Passenger Focus.