Interesting photo showing personnel of the Kent Mobile Reserve. I'd be interested to see the cap badge seen here. Kent is associated with the white horse (in a rampant, i.e. on hind legs, position) and this badge almost certainly includes the horse. The special shoulder title is shown below.
Drop me an email if you know more.
UPDATE: it appears the badge may be the Kent Horse with "INVICTA" scroll beneath. This was the badge of the Queen's Own West Kent Yeomanry that was amalgamated with the Royal East Kent (The Duke of Connaught's Own) Yeomanry (Mounted Rifles) to form the Kent Yeomanry in 1920. See side cap image below courtesy of Jon Mills.
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