. . . . Never seen a double colour light signal like that before. I assume the double signal is for a junction. Was this standard practice for signals at junctions back then. When did route indicators become the norm?
The signal in the photo is a junction signal. . . . . . The colour light signals in that area dated from 1929 and before the Junction Indicator ('Feather') came into use only a few years later, around 1936 on the Southern Railway. Theatre indicators also came in in the mid-1930s.
Manchester Victoria & Exchange stations were re-signalled around the same time (also with Westinghouse as contractors, IIRC) and that installation also featured 'splitting' 4-aspect signals to indicate routes through junctions.
Where more than two routes needed to be indicated (or space was limited for other reasons), then American-looking 4-aspect cluster signals were used in preference to more familiar 'linear' signals.
Examples of cluster signals could be found around Man Vic/Exchange and London Bridge back in the day - but never caught on more widely.
See below for an installation just outside London Bridge Station. You can imagine the sighs of relief when feathers and theatre route displays were invented!
Source:
Borough Market Junction Signal Box at
https://www.wbsframe.mste.co.uk/public/index.htm
One other quirk of those early 'linear' 4-aspect colour lights was that aspects were arranged in a different sequence to the standard seen today. In the early installations, the order of aspects, reading from top to bottom, was
G Y R Y, rather than the familiar Y G Y R. I think a few of the 'wrong way up' LMS-pattern signals survived at Wigan Wallgate into the 21st century on the Southport/Liverpool side of the station.
So, looking at the OP's link to the BBC story - a close observer of the photo might wonder why the side-by-side junction signals both seem to be showing single yellow aspects. In fact they are likely to be early Westinghouse examples, both showing Red.