Back to the original question, and the dieselisation of the former Southern lines from around 1964 was done with a very motley group of secondhand diesel units from all around the Western Region, pretty much one of everything, which were then further switched around as things changed and lines closed. Everything from 4-wheel railbuses to Cross-Country units, based at Plymouth, at an initial dmu depot, a onetime carriage shed at Harwell Street, down the Millbay branch, used for much of the 1960s before the diesel cars moved over to Laira. There were also various diesel loco workings which came and went, D63xx more than others, but Warships and indeed odd appearances by others.
I never saw a single unit on Taunton to Barnstaple, and indeed only one or two ever at Taunton. I used to make a youthful trip every summer hols down the line (and indeed on to Ilfracombe and back) until it closed in October 1966; it was diesel only for its last three summers 1964-66. It used three Plymouth-based sets all day which didn't seem to cover anything else, all 2-car sets with the trailer removed. One Swindon Cross-Country Class 120, one Birmingham RCW suburban unit, and one Derby suburban Class 116. Note two of the three now had no first class, gone with the trailers, although the line was not shown as second class only, and indeed this was the case elsewhere as well, though for the handful of passengers carried it didn't matter.
There was a group of some nine 3-car suburban units transferred from Cardiff, which as the units were assigned indiscriminately were put on some long runs, notably a Salisbury to Ilfracombe service, which led to complaints about lack of toilets compared to the old SR steam-hauled stock, and started a campaign which worked up through local MPs, the Minister of Transport, the BR Chairman (possibly still Beeching), and then came down the instruction to "do something". There were also at Plymouth the nine late-production Swindon Cross-Country Class 120, with two of the three cars having toilets. So over a couple of weekends the two types exchanged trailers, giving 18 3-car units all of which had one toilet. The result looked ridiculous, with the different body styles and all the now useless gangway connections locked and hanging loose against the suburban cars. These sets can be seen in various photos, and indeed I recall my trips to Ilfracombe used them.