Old newbie...

You can, of course, model more than one era if you wish, if you are careful to have only stock from one era on the layout at any one time, although it is inevitable that there will be some details on the layout that may not be 100% accurate for each period. My models are Caledonian and Highland Railways circa 1922, LMS late 1930s to 1947 and BR steam era (with some diesels). This approach isn't for everyone though.

I would second the comment as to the desirability of owning the book 'The Big Four in Colour'. It should be on every British Big Four era modeller's library, and even those modelling the early years of BR.

As for what era to model, if I hadn't headed in the direction I have, with the major part of my collection being vintage, mostly Hornby Dublo, equipment, I think I would choose the years immediately following the grouping, with the possibility to mix pre grouping and immediately post grouping liveried locomotives and rolling and the ability to have locomotives from different old companies but within the same new company running together (as an example, something as bizarre as a Kirtley 2-4-0 running on the old Highland Railway actually happened). The years immediately following nationalisation would offer similar opportunities.