Many British cycling fans used their bikes for double duty: both touring/transportation and racing. Typically, a racer would ride to an event with a derailleur and gears, then strip it down to a single speed for TT or a fixed gear for the track. A cable stop on the CS is one less part to attach and the derailleur would have it's own hanger since so many were proprietary. The Hetchins with the Sturmey Archer hub is weird. All the British bikes I've seen with a SA hub route the cable across the top tube and down the seat stay.
Michael Allison
New York, NY