Ok here is my new reasoning as to why British stop lights go yellow both ways.

Early stop lights needed to be switched.

A simple 4 pole rotary switch driven by a clock motor could control an entire intersection's worth of lights. Connect the n/s red lights and the e/w green lights to the first pole, all yellow to the second, the e/w red and n/s green to the third, and all the yellow again to the forth.

It sort of fits the tech available when stop lights of this type were invented.