However the bus industry tends to use the term "standard fare" (a misnomer if ever there was one) for their equivalent of a Penalty Fare.
A deliberate misnomer to avoid the need for Penalty Fares related legislation, or similarly to avoid being accused of imposing an illegal fine; the principle is that the bus fare is £10 (say), but by buying it before making your journey it is discounted to whatever is normally charged. As the bus fare can be whatever a bus company wants it to be (it then being up to the passenger if it constitutes good value or not), that strikes me as legal.
I don't know how often (if at all) this is ever actually charged, Edinburgh trams aside (where the "on-board fare" is £10, but this isn't, AIUI, considered fare-dodging, nor would it result in prosecution etc, it is simply the price you pay for a ticket on board if you don't buy before boarding; if in a real hurry you may actually consider this good value).
(Edinburgh trams are an odd one - they all have conductors but these are only allowed to sell the £10 fare. If you have them, I don't see why you wouldn't simply have them sell all tickets and save the cost of the platform machines - or indeed not have them and have proper revenue squads etc)