Having had a quick trawl around the nearest I could find were Victorian era USN uniforms, although the Victorian RMLI amd RMA wore similar flat caps but not double breasted jackets. If they're yanks it would explain the tache's.
Are you sure they're all RN? The officer sat on the left could be RN posing with a bunch of yanks.
I'm wondering whether this is a photo of a representative of the Royal Naval Air Service sitting with members of the Royal Flying Corps. I can't help wondering whether, just out of shot, there are aircraft.
A researcher said that the unifroms were not RN in WW1 due to a number of things (haircut, shoes and headgear). But the cap badge they are wearing or the medal and medal ribbon bar both older men are wearing could be the key. Any suggestions?
I think that photograph predates WWI and by a fair margin. The quality of the photography suggest much earlier than 1915. The style of dress, note the long tunics, suggests Victorian era as do the haircuts and the facial hair of the sitting character on the right. The caps worn by the standing figures are certainly not army pattern "Broderick Caps" (introduced in 1902 and phased out by 1910 except for the Bootnecks who carried on with them until, at least, 1914) - they are much too wide and floppy.
I suspect it is a photograph of some "Volunteers" on a training camp. The Rifle Volnteer Movement started up in 1859 as a response to a French Invasion Scare. Volunteer formations sprang up all over the country, each founded by either the Lord Leiutenant of the County or by a Local big wig. There was no national standard uniform and each formation wore what it founder fancied (volunteers had to pay for their own weapons, uniform and kit). The Volunteers gradually came under state control and pretty much finally disappeared after the Haldane Reforms introduced the TA.
There was an Oxford University Rifle Volunteer Formation and at least two Oxfordshire County formations. There are two shooting competition trophies from the 2nd Oxon Rifle Volunteer Regiment in the Soldiers of Oxfordshire Museum, which is actually in Woodstock. You might want to run your photo past the museum, they seem a friendly bunch The Museum - Soldiers of Oxfordshire.
For what its worth, I ran your Great Grandfathers name through the Navy ratings records at the National Archives but did come up with anyone of that name who was born in Woodstock. However, there are about 40,000 naval ratings whose name have never been included in the index (those issued service numbers with a CS prefix). I also checked the Navy list for 1915 without success.