Note that the subject has no medal ribbons which might suggest that the photo was taken at least a few years after WW1 and before mid-WW2.This tends to fit with the observation about the style of the studio background,
The cap doesn't look like the peaked cap worn by interwar other ranks and I can't find any evidence that the Royal Signals had a coloured hat-band in that period.
Even ignoring the apparent Star of David on the studio prop, my immediate reaction was "Police" and, as others have observed, the Palestine Police does seem the most likely.
Maec