Based on a review of markings in "The Broad Arrow: British and Empire Factory Production, Proof, Inspection, Armourers, Unit & Issue Markings", this is likely marking for one of the 28 Battalions of The London Regiment, which was formed in the British "Territorial Force" in 1908 by amalgamation of various earlier Volunteer Rifle Corps units. (The 7th Battalion was the former 3rd London Volunteer Rifle Corps, first established in 1861.)
Territorial Army units were volunteer reserves, and as such would have been armed with various configurations of "Long Lees" at that time, although the Regular Army was re-armed with the SMLE introduced in 1903.
(Has this rifle been converted to charger loading, by any chance?)