Very often I don't want to return. I can fly to Manchester, visit family, take the train to Derby, visit friends, continue to London and fly home.
I can't book advance tickets as the plane might be late, and I can't use a return journey ticket.
It's all ridiculous, and I don't understand how anyone can defend it.
Here in Denmark I can travel tomorrow (leaving at 02.35 if I want to, good luck trying that in England...) to Århus for about £50, any time of day. That's 300km, London to Derby is only 200km.
Booked in advance I can roughly halve the fare, but I don't feel punished if I'm unable to commit to that.
it's a ultimately a difference in philosophy
UK governments over the past 30 years have decided that passengers should pay to run the railway, rather than the taxpayer out of general taxation
if you look at the distribution of earnings of those who use the railways you could even call this a progressive policy
meanwhile most European countries chose the other way round
(HMRC rate is 45p/mile, which constitutes insurance, capital cost, wear, petrol, etc)
the single pricing is because they don't want to sell singles at all, but remember that return lets you come back upto a month later (your choice when)