It's the economics of supply and demand - Virgin only offers its really cheap value fares on the trains that it is unlikely to fill otherwise. Hence why they are rarely available on Saturdays and especially during holiday periods. They know that they'll have no problem selling the tickets. Also, even if you could travel mid-week you're unlikely to find them available for anything other than trains running in the middle of the day because they have no problem filling the trains that give you a whole day in London with people travelling on business.
Virgin is able to advertise these really cheap tickets because the small print makes it perfectly clear that these fares are only available on certain trains and in limited numbers. But they are only available in anything other than really small numbers on the least popular services.
As for cheap day returns from places closer to London this is because the bulk of traffic on those routes are the Monday to Friday commuters and again, operators offer really cheap fares to fill up the trains the rest of the time.
Yes, this does disadvantage passengers who for whatever reason can't use those less busy trains but this is one of the inevitable consequences of the way the trains were privatised. Trains are not a social service, they are businesses run for profit by the operators and the Government. I'm surprised that GNER are offering any cheap tickets at all given that they had to promise to pay the Government £3Billion in order to keep the franchise to run the East Coast line services into London in the first place. The loyalty of these companies will always be first and foremost to their shareholders and, yes, they have to keep the passengers happy enough to still keep travelling but only as long as this is still compatible with making a profit to pay a big enough dividend to keep the shareholders on board too (sorry about the pun - couldn't resist it :-) ).
In the case of any services into and out of London they know that the bulk of the business and hence the profits are made from those who have no option but to travel and can be touched for much higher fares than the leisure market will be prepared to pay. Families who fancy a cheap day out in London come quite a way down the passenger pecking order and frankly are seen as the passengers of last resort when seats can't be filled any other way. You can bet that Virgin aren't running empty trains - the ticket prices would fall if they were - Virgin has a very sophisticated pricing system and you might't be prepared to pay their prices but others clearly are, hence why Virgin feels there is no need to reduce their prices.
In your circumstances it might actually be cheaper to book tickets on the less popular trains running later in the day and book somewhere to stay overnight. Places that rely on business guests often offer much cheaper deals in the summer when their business tails off a bit due to holidays and for Saturday nights all year round because the bulk of their business is Monday to Thursday. For example, Premier Lodge has been offering some really good deals via their £26 pound per room per night offer which works out really cheap for a family of 4 if they don't mind all sleeping in the same room (most have a double and a double sofa bed in them).
SM
and since all the trouble in London and not getting any visitors, or shoppers from the local people who are staying out of London, THERE ARE LOTS OF DAY RETURNS NOW on the Virgin trains available for us northerners to get away for a day, and spend our well earned cash
