Splitting is often cheaper than buying a through ticket. This is especially true if you get a Network Railcard, which gives you one-third off between all stations South of Banbury or Bedford. If I'm travelling between Portsmouth and the Midlands, I buy a ticket to one of those, depending on where I'm headed, and another for the rest of the journey. If you don't have a card, it can still often be cheaper to split, but they are worth having as you get the cost of the card back in a couple of journeys and you are quids in after that.
moneysavingexpert.com has a bot to calculate these for you i believe. I have never used it though but pretty sure Lewis spent a fair bit of cash modelling it to drive punters to his site.
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