In a strange sort of decision, the Modernisation Plan envisaged steam heat boilers only on the new diesels, but electric heat only on the electric locos built at the same time. This went with the later builds of Mk 1 stock having dual heating, and being concentrated on the WCML where this was needed for the electric locos.
I believe one of the issues was the diesel engine industry was unable to come up with adequately powerful units, the operators were scrabbling for every horsepower, and didn't want to give any performance away in having the generator tapped for heating. Gerry Fiennes seems to comment on this in his books. Can you imagine a Class 40 on front line express service over Shap having its power further depleted by heating. The WR hydraulics, of course, had no generator to tap anyway.
The Southern Region did (typically
) pursue an independent course, and never ordered a diesel with a boiler, and had their stock fitted for electric heating, so once steam locomotives were gone in 1967 they were free of it, except for inter-regional services, which other regions thereafter had to supply the locomotive for. Notably, because of the absence of the big boiler and all the associated parts, they could put an 8-cylinder Sulzer in the Class 33, in the same body and at the same axle weight that only a 6-cylinder version fitted in the Class 25-27s, which seemed to offer benefits all round, but it took the rest another 15-20 years to come round to that approach.
The Scottish Region had a go at auxiliary generators in the early 1970s, on the Edinburgh-Glasgow push-pull, with small Deutz diesels, and these certainly had their own issues with noise, failures, overheating and indeed fires (let alone then failures of the fire suppression systems!) which I have written about elsewhere here from some personal experiences.
At first there were also issues in getting enough electric (dual) heat stock together, and some steam generator vans were actually built, and used behind AC electric locos. They were not sufficiently automated and needed their own fireman. They were only needed in winter, as in those times, instead of being temperature-controlled, heating was to a fixed timetable - on from 1 October, off 1 May. CIE in Ireland, I believe, did this as policy, didn't put boilers in diesels, and for long ran with heating vans, right through to air-con stock.
Steam heat on diesels was a thorough nuisance and prone to many sources of failure. The boilers themselves were not really traction standard, and got a range of issues, especially being unable to light. BR's specification of the cheapest spec for fuel didn't help. They also needed a range of auxiliaries, there needed to be pumps for both oil fuel and water from the underframe tanks, and these introduced their own unreliability. The water tanks were typically cheaply made and would leak, the water in them would freeze in winter, etc. O S Nock got his last big steam run on the WCML, with a Duchess from Crewe to Carlisle, when the Class 40 he was footplating and timing had to come off at Crewe with just this fault where the tank had leaked to empty.
It was all so easy in steam loco days, when you just tapped the boiler and such problems were unknown. It was also apparently so easy over in the USA, where they pursued a similar course with steam supply even longer, with new build boilered locos well into Amtrak days, not only for heating but also for air conditioning (yes, steam driven air-conditioning was something the USA cracked reliably and universally from way back in the 1920s). This was in bitter winter conditions where heat failure would be something far more significant than in Britain. Maybe the easiest thing BR could have done is order some of the boilers the US manufacturers fitted.
The story I liked best was in Modern Railways in the early 1960s, about the morning Yorkshire Pullman, when it was 50-50 whether steam or diesel hauled, with notable difference in how well the train was heated. The breakfast chef would look out just before the start to see what was on the front, and if a diesel would prepare much more porridge.