The answer to the problem is simple. There are enough "through" stations on the British rail network to use as a STARTING station on a leg, which offer long journeys whichever direction you take.
Stations such as Preston, Crewe, York, Newcastle, Derby, Salisbury, Reading, Bristol, Edinburgh, etc, immediately spring to mind. I would describe all of these as "through" stations...see what I am getting at? All of these confirm to the strictures of Rule 1 on the matter of "through" stations.
Why bother starting a leg on a small branch line (with a terminal station) in the first place, as this only offers one reasonable direction of travel on a leg.