A few questions come up here.
There is a comparison to supermarket pricing. However supermarket pricing is vastly more simple. But does it need to be? Why not price every stop at a certain price, so for example Leeds to Huddersfield is x, Huddersfield to Stalybrudge is y, and Staybridge to Manchester is z. Therefore combined price is x plus y plus z. Too difficult?
Supermarkets mess things up too when they try to complicate matters. I remember 2 for £5 deals on chicken meals in Co-Op. What really happened was that as each meal was £3.50, they'd takeoff £2 if you bought 2 of them. So when they knocked down individual prices at the end of the day on individual meals to say £2.50 each, you could get 2 meals for £3. I'm sure they didn't intend that..
Some questions were asked:
"Explain how 'the cheapest split' could be offered for London to Glasgow, in a way that any person could easily understand it."
That would be unnecessary if individual station to station stops have fixed prices.
"Explain how that same system could make it clear what the restrictions and limitations are."
Ditto.
"Explain how the system could cope with people who do not want to be tied to specific trains, dates or routes."
They can buy the right route for the journey/s they wish to make.
I spent 20 minutes this morning wasting time booking Leeds to Sheffield, Sheffield to Derby, Derby to Birmingham, Birmingham to Cheltenham, Cheltenham to Swindon. That is not an efficient use of anyone's time or resources.