One of My Holy Grails...

Never a truer word spoken, the locomotives are only the beginning, followed by the various wagons, carriages, etc...  Not to mention all the ancillary stuff like scenery, accessories, buildings, electronics, etc...  It can become a lifetime obsession very easily.

Myself I can't be too bothered to concentrate on a particular Group to model, I'm more interested in which locomotives and rolling stock I like and have no issues with mixing and matching them on a single layout.  Of course I could take the easy approach and go for a BR era, but I like some of the really old stuff that never made it to that timeframe or have simply not been modelled in BR liveries.

Ultimately its whatever makes you happy that counts!