In my experience, if a traveller arriving late goes straight to that guard and says 'Sorry, but my bus was late and need to get this train to get to work on time, can I get a ticket from you please?' the answer will rarely be anything other than 'Yes'.
Didn't we have a thread a little while ago in which many people suggested that the above question was fine but that if the guard then refused to sell anything other than a full price single/return then the passenger could have no comeback because they hadn't been sufficiently detailed in their questioning? i.e. if you have a railcard then you need to clearly say so, etc.