Looking at BRfares there should be an Off-peak single @ £11.90, however only the Anytime single @ £21.00 seems to be available to buy. So what has happened to the former?
This is a reminder of why BRFares is a useful tool, but one that is only useful if you have the requisite knowledge to make use of it!
The Off-Peak Single fare you see for £11.90 is what used to be known as a "Saver Half", or nowadays "Off-Peak Single (Online)". It is a one-way ticket but that can only be sold in conjunction with a return journey and can only be booked online. It is designed for passengers who wish to combine different ticket types on their outbound and return legs - as for example they may be travelling before 09:00 in one direction (and therefore require an Anytime ticket for this journey), whereas they may be travelling after 09:00 in the other direction. For many other journeys they would have no choice but to get an Anytime Return, as that is the cheapest option despite the return journey being eligible for an Off-Peak ticket. But for Virgin Trains-set "flows" passengers have this choice.
It is not quite as good as an ordinary Off-Peak Return in that the Off-Peak Single (Online) is only valid for two days from the initial date of issue, rather than the whole month which an ordinary Off-Peak Return's return portion would be valid for. But nevertheless it is good value and it demonstrates that single-leg walk-up ticket pricing
can work for the rail industry!