On another site, with a much easier booking system, the cheapest fare for two is £79, which makes it almost affordable. That system allowed you to specify if you have railcards, but you had to choose a train time both ways.
Book two months ahead, though, and a one-way ticket, per person, can be had for £15 - £60 return for two, if you can fit in with train availability.
Spontaneity doesn't come cheap, where rail travel is concerned. But spending 9 hours on the coach, however little it costs - though "FROM £12" could mean anything - is not an option.