I have the book Memoirs of a Railway Engineer by Edgar J Larkin who worked on a lot of the BR Standard designs. From memory Larkin observed that while the standards were being produced, steam was expected to last into the 1980s, but it seems unlikely that with a huge number of steam locos both built by BR and its predecessors on hand that any formal date for the end of steam had been adopted. It probably isn't until the Beeching era that it became clear that traffic would be shed at such a rate, that the railway would soon have enough modern traction to be rid of steam fairly rapidly. In the event, the decline was so rapid that it did not just enable steam to be jettisoned, but the less successful modernisation plan diesel types as well.
Had the closures not been as dramatic, retaining at least a strategic sense that railways would remain useful and the modernisation plan more measured with longer to find diesel types that lived up to expectations of efficiency and reliability, then a trajectory more reminiscent of that in West Germany might have been the eventual outcome, with more modern steam classes persisting until the mid to late 1970s, but their poor labour efficiency in a world of high wage inflation would have brought about the end sooner rather than later as it did across the rest of Europe. The same process occurred in China more recently, where similarly large numbers of standard QJ, JS and SY steam loco types built in the 1980s in a coal rich nation with cheap labour, had virtually disappeared by around 2005 (apart from a few lingering mining operations) as labour costs (and pollution concerns) have risen.
Where steam persisted for longer in Europe, for example in Poland, this was probably the result of a combination of almost no funds for investment in renewals, coupled with a continued political requirement not to close anything, so low traffic lines simply had to carry on with what they had. Once line closures started in the early 1990s, steam dissappeared here too in just a few years. In East Germany, the narrow gauge lines survived just long enough to become (at least partially) heritage driven operations (as did the Wolysztn steam depot in Poland).
