I would like to be the first to welcome you AND your Lee-Enfield to the forum! Welcome aboard!
What you have, friend, is not a scarce civilian sporter, but something which is much more significant and vastly more interesting: you have a genuine First World War combat rifle - the toughest ever made - which went through BOTH World Wars before it was retired and sold off as surplus and only THEN was stripped and made into a sporting rifle. The rifle brings with it its entire history in outline form; this is all in those cryptic little markings which you have so nicely photographed. Let's take a look at some of them.
To begin with, the markings on the Butt Socket you have already remarked upon. They are the BASIC information about your rifle but they are also proof that this was a military piece; a commercial Sporter would NOT have the Royal Cypher on it (the Crown and GR) because that only went onto rifles which were built for His Majesty's military service. It was, as you know, built in 1915, so it is now 98 years old (happy birthday!) and it was built at the Royal Small Arms Factory at Enfield Lock, which is where the entire modern age of technology was born: right in that one factory. It was the first factory in the world to be able to turn out a complex piece of equipment (in this case a rifle) which was 100% interchangeable...... and do it with machine tools. When built, it conformed to the Sealed Pattern for the Short, Magazine Lee-Enfield rifle, Mark III, which means that it was built with a drift-adjustable rear sight, full wood, Magazine Cutoff and a set of Volley Sights, all of which have disappeared in the following years. If you can find the parts, you can restore this one to its original glory OR you can restore it as what it turned out to be in later years, which would have been a Mark III* (Mark Three Star) which never had the Star banged into it. Either will be correct, depending on period. Either way, with 17 inches of Wilkinson steel on the front end, you have the most brutally-efficient battle rifle ever made, anywhere, any time.
The ENGLAND on the left side of the Butt Socket would have been applied after the Second World War, when the rifle was sold as surplus.
The markings on the Chamber are an interesting mixture and tell you a lot. For one thing, it appears that your rifle had a complete rebuild with a naw military (the Arrow) barrel in 1918, which is when the more-delicate original rear sight would have been changed-out. The ".303" 2.222" 18.5 TONS/" square" was applied post-World War 2 when the rifle was exported from England. These markings tell you the bore diameter, the length of the casing and the working pressure of normal ammunition for the rifle, along with the assurance that it was TESTED for this pressure. The BNP and Crown tells you that this work was done in Birmingham..... but the rifle was built originally just outside of London, which is where the original MILITARY proofs were marked: the little crowned crossed flags and P.
If you want to see what your rifle was like the day it was built, there is a photo "tour" with about a zillion pictures in the Military Knowledge Libraryon THIS forum. There is ALSO a photo "tour" of a later rifle, which is how yours ended up.
Original 1915 rifles in ANY condition are scarce in this country. Production was not as high as it would be in later years..... and many, many rifles were lost, captured, scrapped, destroyed during the 3 years following being made. And then this whole process was repeated, 20-odd years later, when the old rifles were hauled out of storage and expected to serve for another 6-1/2 years of war. Very few survive today. You are a lucky guy.
So, what you have is a very REAL chunk of HISTORY. Think of almost ANY battle of EITHER World War...... and there is a chance that your rifle was THERE.
It doesn't get much more REAL than that.
And a nice point is that, while you re looking for the parts to restore it, the old girl will keep your deepfreeze filled. The old .303 will flatten anything in North America with 1 shot.
Your are a Lucky Guy!