The last steam loco's on BR

Looking at my 1968 Combined Volume - OK, it was given to me many years later not one I was using at the time, the surviving loco's are dominated by Black 5's & 8F's.

Clearly there is a certain logic to this but why did so few BR Standards make it to the end? Only a handful of Britannia's 5MT's & 9f's are listed despite being capable loco's and of course newer.

 

The programme of dieselisation was by area and the arrangement was that steam locomotives in a dieselised area were either transferred to another area or scrapped. Given that the Black 5s and 8Fs were a Stanier design hence had many common parts it made sense to retain these in order to ease spares supplies whilst there were increasing fewer areas to transfer other classes to. Note also the effect of Beeching whose line closure programme released both steam and diesel locomotives hence reducing the number of lines that operated with steam traction.