I commuted from first Westbury (September 1980 to early 1983) and then from Bristol Temple Meads (until 1988) to school in Bath, mostly on these workings and can confirm at least during those five years, the vast majority of the arriving locos at Bristol were re-engined with different loco for the Temple Meads to Cardiff leg, with class 33s almost universal on the Bristol to Portsmouth section and working a good 85 to 90% of the Bristol to Cardiff runs, though these were a little bit more likely to get a class 47/4 or later 37/4. Given that these were not air-conditioned trains, no heat 37s or 47s were very occasionally put out on the Cardiff legs in the summer, but it was the Bristol to Taunton loco hauled peak trains and the afternoon Bristol to Weymouth train that were more commonly where the interesting locos would appear, plus towards the end of the period increasingly assisting DMUs with out the correct number of working engines.
Class 205 DEMUs were a very occasional visitor, but I can't remember ever encountering 33/1s with 4TCs - even the 8 car sets of Southern Region air braked East Grinstead line Mark 1 commuter sets were more common, particularly at weekends, largely made up of TSOs where the Western Mark 1s tended to be SKs and CKs.
My top working on a to / from school journey was on 19th September 1983, when having not even seen the loco, I heard an engine sounding somewhat different to the normal class 33 and went to the front to see 25069 had taken my morning train, 1O65 0645 Cardiff - Portsmouth forward from Bristol to Portsmouth. I was too conscientious to skip school to go all the way down and back, and had to be content with getting off at Bath and seeing it from the School playing field that overlooked Bathampton Junction on the return working (the 1210 Portsmouth - Cardiff). Watching the trains was always an improvement on Rugby!
