Not all Brit rifles are SMLE's. Only No. 1 Mk III's are. SMLE's. No. 4 Rifles are not. Neither is a No. 5(Aka Jungle Carbine. That's an American marketing term.). Which is a slightly modified No. 4 with a shorter barrel and lightening cuts on the receiver. And had nothing to do with jungles.
No parts, even mags, will interchange. Butt stocks look the same but are not. Cuts where they fit the butt socket is slightly different.
No. 1 mags have 2 locking lugs on the back and will not fit a No. 4. A No. 4 mag has one lug(also fits a No. 5). A No. 1 Mag can have the lower lug ground off to fit a No. 4 and still sort of fit a No. 1. Works, but not well.