Crewkerne to Bristol Temple Meads is valid via Exeter or Salisbury. With the hourly service each way it means that you only have half an hour to wait. As such it does not matter which way you go. However it is tempting to do a circular journey.
Unfortunately, as was highlighted in another topic, there is no valid route via Yeovil Pen Mill.
FOSS▸ rover gets round the routing issue so you can go via pen mill ... also valid via salisbury, unfortunately not much use via Exeter unless you go for a bus trip between taunton and crewkerne