So BR had spare stock (more likely on a contracting railway)...do you think DfT would allow that now? And anyway there is spare stock now, but rules have moved on and you can’t just roll out any old crap that is lying around somewhere.
BR could also just take stuff away from other routes without bothering about service commitments on those routes. Just do what was most convenient and cheapest for BR.
I think BR had plenty of spare stock. It was old stock that had been replaced on more important routes by newer stock. A lot of old stock was scrapped but the better examples were kept as backup and for summer Saturday extras. These old coaches could be formed up with a locomotive as and when required. My feeling is that, being coaches, there was not much to go wrong with them if they were left idle in sidings for weeks/months. As freight was declining there were lots of locomotives available. there were also a lot of sidings in the past where stock could be kept.
The parallel now would be keeping any Pacers that are in good condition as backup. But we have too higher standards to let that happen !. Of course multiple units have more to go wrong with them if they are left idle for too long. We are going to see a lot of serviceable EMUs left idle in the next five years btw.
Another process that could help is a steady replacement of fleets. At the moment we are stretching manufacturing by ordering a large amount of stock. This in turn will need replacing 30-40 years down the line all at the same time. We did it when we replaced steam with all those DMUs in a hurry. We did it again when we held off replacing those first generation DMUs until all were done quickly. It could have helped that the Pacers were not expected to last as long as the 150s but the Pacers have hung on for so long that the sprinters cannot be far behind in needing replacement.
It is alarming to see how many 80Xs had been ordered when the first ones had hardly done any work. No proof they were capable - just promises. We have ordered so many that Newton Aylcliffe cannot cope so we are importing them. There will then be a lack of orders and Newton Aylcliffe might well close down. That is the sort of thing that happened under BR days anyway.
The electrification boom and bust cycle is just about to go South as well. The common factor is government control. That has been the same since BR was formed. Only difference is that under BR the state recruited/chose the higher management and held the purse strings. Under privatisation the state micro manages the railways and still controls the finances via subsidy.