As you all know, in my alternate timeline steam remained prevalent worldwide for many more years than it really did. But what would happen to the RWS in this timeline. Since Diesels made a good issue in latter books.
Here's what I'd do....
In this timeline, Standard types steam engines built by BR would replace engines from Southern, LNER, LMS, and GWR. Claiming themselves to be the ultimate evolution of their species. While older engines like the Fat Controller's are just primordial forms of life.
Diesel could be the proposed 0-6-0 shunter:
D199 could be replaced with a British Railways standard type 4 4-6-0:
Bear could be the propsed 2-8-2 version of the 9F (which would probably be called 8MT if it was still built:
BoCo could just be a BR standard type 5 4-6-0:
Any other ideas?