I'd argue that the majority of the type 2 fleet falls into the category of being built without a need. The premature scrapping of diesels (aka "standardisation") in the late 1960's and 1970's shows this well. The type 2 fleet simply became the backfill for the scrappings.
Then you have the class 15, much of the diesel shunter fleet, the Blue Pullmans, Hastings line units (converted to tadpoles), arguably the Tyneside EMU fleet (line rapidly de-electrified). And as noted above, much of the DMU fleet