To be clearer, splitting tickets is legal, and while the second ticket may become invalid if the first train runs late, train companies now honour the second ticket if the reason you miss your train is because the incoming train has run late.

However, what can be in breach of the terms and conditions is to buy a ticket for a longer journey than desired and then get off at an intermediate station. If you read the comment to which you're replying, it's the "overlength ticket" that he is saying can be illegal.