Outside of "Sniper Rifles", which are very valuble, as an investment, Lee Enfields have never increased in value as much as American Rifles, or Nazi rifles. I suspect that is because Americans like their home grown rifles, or the rifles they see in the hands of the compotent "Bad" guys in WWII movies.
There was a massive importation of surplus rifles before the 1968 Gun Act. In the middle 60's, Lee Enfields were at $12.00 , and 03 Springfields from $30.00 to $40.00 (if memory is right). Twenty years later, before the great importation of the late 80's, I would see Lee Enfields priced at $90.00 and M1917's and Springfields at $250-300. You never saw a Garand, so I have no idea what they were worth.
While the prices of all Mil Surplus keeps on rising, I will predict that Sniper dude guns, American guns, Genuine Nazi guns, will always outpace all other mil surplus rifles.
I believe that Lee Enfields will keep their value above Nagants, french MAS's, Italian. The Arisaka rifles keep on increasing in value, and they are bringing better prices than Lee Enfields, which surprises the heck out of me. Used to be no one wanted one.
Of the rifles I have handled, British No 1 Mk III were not valued as much as No 4 Mk 1's. The best built Mk III's were Australian, in my opinion, the WWI Australian rifles are extremely well made. British were next, with Isapore third.
When you get to WWII, Savage made No 4's have a lot of tool marks, but because they are American built, I think they will have a higher value than any other. The absolute nicest finished No 4Mk I*'s are the Canadian Long Branch. These rival commerical actions. The British built rifles are excellent.
The best, in my opinion, are the post war No 4 Mk II's. These rifle are well built and have all the improvements.
The 1960's Isapore's are crudely built in comparison to the Isapores built under the Empire. However, they will go bang and are interesting rifles.
The handiest Lee Enfield is the No 5, Jungle Carbine. Everyone needs at least one. That rifle is one of the very few military surplus rifles that would also make a good hunting rifle, as is.
If you want a Lee Enfield, just go buy it. There is nothing wrong with a Lee Enfield, I have always thought that the No 4 Mk I, or Mk II was the best battle bolt rifle ever built. That ten round box magazine gets five more votes than any five round magazine, the sights are excellent, the action slick.