Don't forget that Gresley designed the prototype of what became class 76, this would have taken the place of many of the B, K and even the V class steam locomotives. The ECML would probably have been operated by a streamlined version, perhaps 'A4' shaped with the train 'topped and tailed' like an IC125.
As for the Great Western as they looked towards Switzerland for some new technology one could imagine a 'Crocodile' pulling an express over the Devon banks.