Feature or not, it should not be enabled for journeys where the only benefit is to the company in overcharging its passengers contrary to the Consumer Regulations - and where it was deliberate, such as the Virgin example you suggest, the question asked should explicitly ascertain that that is the reason the passenger is making the selection of the higher-priced version of the ticket. None of the wording on the machines I've seen is satisfactory for the case where the passenger has no pre-existing idea why of they are being given the choice. But this is getting off-topic. Haywain has the right idea.