If there's scope for, say, Amazon to fill a train to send things to Scotland (I think that's where a lot of or even all returns go, where they're checked, sorted and sent either for resale via Amazon warehouse or scrapped*) but not much to return at the time it would need to leave to get back to London, can they not manage things more effectively with extra trains?
That way a train arriving at Scotland can go into a sidings and run back at a more suitable time, when it can contain goods, and there's still a unit in London able to go up the next day. No trains run empty unless there really is nothing to move and the second train is needed at the other end to stop them both ending up in one place. Surely there are goods to move south? What about food? Could these trains have refrigerated containers loaded, with power taken from the train?
In other words, train 1 leaves Euston to Scotland and train 2 leaves Scotland. Both then sit around until ready to go each day, rather than running empty.
Of course there are crewing issues to work out and other logistics, but with plenty of old trains out there there is not necessarily a need to worry too much about running empty as with a lorry. With a lorry, there's a driver that moves his/her tractor around - but different drivers can move around the freight, and the driver now in the wrong location can travel PASS on other trains instead of taking an entire unit with him/her.
* I was shocked just how much undamaged goods are destroyed because it's considered easier and cheaper. I also have no idea where this facility is located in relation to the railway.