Split ticketing means buying two or more separate tickets to cover different legs of the same journey, rather than one through-ticket. The start and end points of your journey stay the same, but instead of paying for the whole thing in one go, you split it at an intermediate station.
It's completely legal and widely used across the UK. Split ticketing is explicitly permitted under section 14 of the National Rail Conditions of Travel, the official rulebook for UK rail travel. National Rail has a full guide on how it works here. You're not exploiting a loophole, it's an officially recognised way to buy train tickets.
