Well Boo, no wonder I couldn't find it! I was searching Army ranks - the actual uniform looks like khaki, although I did wonder about the anchors! Thank you.
As indicated by B00's link, the arm badge signifies a Petty Officer in the Royal Navy and has existed as Crown and Crossed Anchors since 1857.
During WW1 the rank was PO 1st Class (PO 2nd Class had disappeared by 1907). The '1st Class' designation was later allowed to disappear such that the rank is now just PO.
The uniform would be a better signifier of date than the badge.
An RN rank wearing khaki possibly points to the 63rd (Royal Naval) Division, formed in August 1914 from naval reserve forces to supplement the army. The RND was retained under Admiralty control even though they were fighting on land alongside the army. The RND was disbanded in 1919.