In fact there was very little that was only used for the stereotypical "10 trips a year", and what there was on summer reliefs etc was distinctly marginal in its costs. You also found the "holiday stock" was what was turned out for Sunday excursions, midweek "mystery excursions" from the resort (typically on Thursdays, by which time boredom had set in, and every year the PR team found someone for a press story about one having taken a couple to their home town), Christmas week reliefs, specials to the Motor Show or the Ideal Home Exhibition, etc.
Fact was that BR costing systems of the era (like other pre-computer age industries) just averaged vehicle costs across the whole fleet. There was no analysis of usage of light bulbs or brake blocks by vehicle. Depreciation of the older vehicles used was typically written down to zero, and there was nevertheless a general understanding of "marginal costing". It was however good for the mainstream operations like the early Inter-City services, as it masked their substantial additional costs, and helped them to justify buying new stock. It is disappointing that Stewart Joy, Beeching's Chief Economist, never got a proper costing done of a sample of the different uses stock was put to.
Nowadays we have different inefficiencies. We have time after time trains incapable of service in the first 18 months after their delivery (in the 1950s stock was in use the day after it arrived, and it worked). We have MML services twice a day from Leeds to London which departed daily at 0530 and 0630, and which thus left there as empty stock. We have Orcats raids with trains which run back and forth empty but pulling in revenue from passengers who all have gone on a different service. And so on.