As a guard, if a passenger is travelling from a station where there were facilities to buy a ticket and they approache me to buy a ticket as they didn't have time before boarding, I will sell them the full range including group or rail-card discounts.
If I find them sitting there without a ticket, even if they then say they want to buy, then they are trying to fare evade (in my opinion, which is the only one that counts in those circumstances), and they will have to give me a VERY convincing reason that they don't get charged an Anytime single, or referred to Revenue (in the very unlikely event that I have Revenue on board).
In both cases, if appropriate, they will be advised they have potentially committed a criminal offence and could have major problems if Revenue were on board.