B Row L-r Valentine. Percy, Francis. F Row Victor Cyril Rupert

Rupert was an Artillery Signaller like myself, you can see his Signaller crossed flags on his left sleeve.
Great historical photo mate

Thanks for the Andy. Is the badge of the Royal Garrison Artillery? have a look at Valentine.

Both Valentine & Rupert have a Royal Artillery Cap Badge

The Royal Garrison Artillery (RGA) was formed in 1899 as a distinct arm of the British Army's Royal Regiment of Artilleryserving alongside the other two arms of the Regiment, the Royal Field Artillery (RFA) and the Royal Horse Artillery (RHA). The RGA were the 'technical' branch of the Royal Artillery who were responsible for much of the professionalisation of technical gunnery that was to occur during the First World War. However it was originally tasked with manning the guns of the British Empire's forts and fortresses, including coastal artillery batteries, the heavy gun batteries attached to each infantry division, and the guns of the siege artillery. The RGA were amalgamated with their sister arm the RFA in 1924, from this point onwards the only two arms within the Royal Regiment of Artillery are the Royal Artillery and the Royal Horse Artillery.