TOPS 67-82: The first post-diesel era where BR blue come to the fore. The book 'Modelling the British Rail Era' is a great resource for this period - multiple units, and the successful diesels from the earlier nationalisation period dominate. Most freight becomes trainload and almost all wagonload freight is finished.
82-95 Sectorisation: British Rail divides into multiple sectors (containers, construction, civil engineering, intercity, etc.) and a huge profusion of liveries after 30-odd years of BR Blue. Wagonload freight has a slight comeback witth the short-lived Speedlink service. This is extremely popular now on Youtube (everard junction, dean park, etc.) because most people in their 30's who are starting to have disposable income can remember it.
95-today Privatisation: A complete shit show of trainload freight and ever more shiney 8+ car multiple units. Basically unmodellable unless you crave a procession of stock through a barren landscape with a bus-shelter station building.
Check out Iain Rice's "Creating Cameo Layouts", he's very firmly in the pre-grouping period but has really interesting concepts about layout design which go beyond the usual dichotomy of 8x4 board vs exhibition behemoth vs loft queen.
Right now I have two layouts on the go, one is 00 set in 1987 in the British Rail southern region - I get multiple units from my childhood (4CEP, 2EPB) as well as locos I can just about remember (Class 33, 37) and rolling stock I can either remember (BR Mk. 1's and Mk. 2's) or can see currently (PGA gravel wagons).
The other is set post-nationalisation ex-LMS in the 1950's in 2FS where the whole layout will fit into a 4' x 1' cube, so I can keep it in my office.
Generally, the earlier you go the more modelgenic reality is, and the more appropriate it is to go with a finer scale (i.e. nobody gives a shit if your O gauge Class 66 has oversized flanges because of the size of the loco, but it's much more noticeable on an SE&CR P-class.