Everyone always says that, I've said it myself a few times, but they always compare a very expensive last-minute train ticket with a cheap plane ticket booked in advance.
Try booking a flight Friday afternoon returning Sunday afternoon on both types of transport the same weekend and you'll find they both take a lot of money off you! If the last-minute rail price was £300 and the air fare a lot less then they would sell no last-minute rail tickets!
I can think of better ways of spending £300 but I guess at those prices it's probably companies paying.
That's an excellent point, plane tickets for comparable times
booked at the same notice
will almost certainly cost even more. To get the £100 plane tickets these days you have to book months in advance.