As davkt said, I think this is down to what was 'at hand' at the Lightweight unit in Nottingham when these were being built
English craftsmanship at it's best
Yup, whatever the dropouts they still makes a great fun ride! I know round the web some folks complain it is very twitchy handling, sure it is more lively than something with a longer wheelbase but its a race frame not a tourer and ride it like you mean it and it is perfectly predictable.
With 28mm gatorskins, cheapest 28mm innertubes I could find and a Brooks B17 mine is only 10.3kg, and it isn't a particularly light build, it was my dream bike when I first started work in the mid 80s but parts were more about affordability than weight! Mostly 105/RX100 with a few other bits and bobs as well.