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Looks like coastal defence to me which would make it Royal Artillery (or, later on, even Home Guard). Could be the real thing or a training establishment. Is there sufficient definition in the original image to dechipher what is written on the life belt, apart from what appears to be "Royal Artillery"? In particular does the original show more of the bottom half of the ring?
Someone more expert than I might get something from the image of the bloke in charge with his Great War ribbon. Cap badge? Uniform - R.A. or even Royal Marines?
Sorry, more questions than answers but let's hope for more discussion on this.
Unfortunately the bottom of the photo does not show more of the ring. However it is a postcard/photo and on the back is printed J C Lawrence & Sons, 22 Elmhurst Road, Gosport. The only other word clearly defined next to and below Royal is "Sea"
Googling reveals some interesting information about this photographer, not least that there is an online archive showing some of the historical material - and, perhaps more significant, contact details. If there is some reference number on the postcard, they may still have documentation describing who, where and when.