The true Enfield experts will doubtless chip in later. For the moment let us just look at the nature of the damage to the wood. Unlike a fully stocked rifle, where the tang banging back into the recess on recoil can cause a split in the neck, the forestock of an SMLE is basically a pre-loaded wooden beam hanging on to the system between the butt ring and the barrel, with the pre-loading being adjusted by the front trigger guard screw. Recoil will not damage it, but excessive flexing will. This excessive flexing could have arisen from grossly excessive pre-loading by some Bubba who though that an upwards force on the muzzle of a couple of stone would be a good idea. - No, I don't seriously think that. But the same effect would be produced by severe warping or shrinkage of the forestock, maybe creating a tight barrel channel that was wedged apart by the barrelled system.
Whatever the cause, the effect is that the wood is split over most of its length. Since an incomplete split varies from x down to nothing, there will be a portion where you will not be able to effectively force in any glue to hold the wood together. In time, the split is thus likely to spread over the remaining unglued length.
All this is just to prepare you gently for the following conclusion: as a matter of good woodworking practice (nothing now to do with SMLEs) the best repair would probably be to deliberately make the crack continue right down to the end so that you see daylight from end to end, and then glue the two halves together again, to create an unstressed solid bond over the entire length.
You can then fettle the once-more solid piece to suit the rifle. Unless, of course, it is too badly warped, in which case a new forestock is unavoidable. Impossible to tell from here.