A good read! I took this route to Plymouth some years back, albeit in Standard Class before I discovered the Britrail Pass senior discount. I then took the branch line up to Gunnislake which is a marvelous ride. Something of a contrast as the speed limit is 15 mph and the train is required to stop at the level crossings. The Calstock viaduct is particularly spectacular on or off the train.

As an aside, East Coast First Class service at weekday mealtimes is very good.