Ah, the long-standing question of what the 'appropriate' ticket should be. I agree that an off-peak ticket is the correct one to sell if it valid on that service. And that's exactly what I've been sold.
Yes, it is a breach of the byelaws and so isn't advisable. However, here's a secret: it's quicker and easier for a guard to sell a ticket than to report you for potential prosecution. So in all likelihood, if you have the right attitude, they'll do just that.
The virgin guard insisted he'd sell me an anytime single, but actually sold me an offpeak when it came down to it. The FGW guard was too busy chatting to the buffet person to make a £130 sale (Don't they get commission?)