1G77 used to run from Penzance to Gloucester (between May 2022 and Sep 2023 I think?). Before May 2023 it was operated by Castle Class HSTs too so that planning might involve getting the set back to Bristol SPM at night.
I believe it ran like a stopper service south of Exeter, as the Devon & Cornwall leg was mainly during after-school/-work hours; north of Exeter it ran as an express skipping Tiverton too. The skipping might be to get it back at Bristol TM in time, so that it can form the (approx.) half-hourly Gloucester departures from Bristol - it's a stopper between Bristol and Gloucester too.
Purely my speculations. Was always a nice service for home (Bristol too) after a day out on the seawall.