I've got four SMLE's. A Mark III no. 1 that has the magazine cutoff that slides over the magazine, a MkIV, a Jungle Carbine and one of those that was rechambered in India for the 7.62 x 51 round (.308). The Indian rifle shoots around corners and keyholes. The Jungle Carbine while nifty looking shoots almost as badly. The Mark IV shoots fine, but god it's an ugly beast and my MkIII shoots fine and is my favorite.
You can easily adjust the head space of a SMLE. If you remove the bolt you will notice that the bolt face is screwed into the bolt body. If you unscrew it it will come out, the firing pin will be sticking out of the bolt body. English armorers kept a box full off bolt faces and using a go/no go gauge they'd afix the proper bolt face. The trouble is that the bolts were removed during storage and you may not get the correct bolt, so your headspace can be way off, but it's easy to fix if you have the right parts.
I think the neatest SMLE is the Mark III that has the WWI volley sights. They'd get up each morning and send a magazine full of rounds over to the German trenches. They figured the bullets were going the right direction and they might hit something. Since it's cock on closing and easy to pull open, it was pretty easy to work the action, but you have to push it home pretty hard. The WWI Tommies were taught rapid fire with them and they could get them to cycle almost as rapidly as a machine gun. Fifty guys firing like this could put out a lot of lead.
Mark VII .303 round made in Dum Dum India had lead cores inside of a gliding metal jacket. Tommies would nip off the ends so the bullets would open up on impact inflicting a wicked wound. Thus dum dum bullets which were made illegal. You can't kill'em that way.
Sometimes you'll find a SMLE with around a foot of wrapped copper wire about half way to the front sight. Those rifles used to be almost free. Really ugly. I've heard two stories why this was done. One was the wire made a handhold to make it a better club upon which to mount the bayonet and the other that the wire made it stronger or easier to hold when shooting rifle grenades. Don't know which is true, maybe both? Now I'd kinda like to have one. I've seen these rifles painted green from the breech forward! Weird.
Most (I think) Mark IV rifles are painted black and not blued or parkerized. Cheaper I guess. I suppose you could removed the old paint with Jesso paint remover and repaint it with glossy black paint. The original paint looks pretty glossy to me, but flat back would look better. No matter how much you paint it however the MK IV SMLE has got to be one of the ugliest rifles ever made.
If you go to Britain and talk to an old soldier who shot one of these rifles he'll wax nostalgic if you tell him you own one. The average Brit can't you see. Poor bastards. Hope it never happens here.