That's much more complicated than it sounds .... the whole of the fares system is full of anacronisms and special deals in dusty corners, with some outrageously high fares and others which are real bargains.   

If you want to go to the London Eye for the day on Saturday starting from Trowbridge, but return in the evening to Keynsham, then you'll want a Shirehampton to Waterloo via Salisbury ticket at 27.10 (I think - can anyone see cheaper?)

Splitting tickets is often a good way or reducting costs - if you're travelling to the Midlands, look at the price of a ticket to Cheltenham and the Cheltenham to (whereever) and if you'r going into Paddington, look at a Didcot split - perfectly allowed if the train calls there.  But the software to do it would be nasty to write, and I can't help feeling that people being sold split tickets might not understand the "must stop at ..." rule.   Visitors from overseas would feel that there was something wrong and might think they were being cheated (I know this - I have had to justify splitting suggestions).

I know that the Association of Train Operating Companies has - past tense - moved to shut down web sites offering automated systems to help people find the truely cheapest way (not ATOCs finest hour IMHO!) but really it's the whole fare system that needs properly sorting out.   Of course, none of the political parties will do or promise that as there would be significant winners and loosers, and the loosers would switch their votes away from the party putting in the proposal if they feared a significant rise in commuting costs.