Definitely a branch of the Royal Artillery, but whether he served with the Royal Field Artillery or the Royal Garrison Artillery or even the Royal Horse Artillery from the cap badge alone is difficult to say. The shoulder titles should help so if you could post a close up of the shoulder titles it might help. The shoulder title appears to have a T above it so he was possibly a Territorial.
If you could tell us his name it would be more useful in identifying his unit,
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