Training rifle converted for the Royal Navy. Notice the 'N' for Naval Service stamped on the left butt socket metal. Sht 22 Mk.II is the model.
An official 'bitzer', it has a later model barrel (SMLE) with sights. The stock was hogged out in the barrel channel and a wooden liner glued in for the thinner barrel. the wood was bobbed in length and a new nosecap fitted. The volley sights are left over from the original long Lee.
Your mag looks to be a WWII cadet C No.7 type, which is just a loading platform and empty shell catcher.
Great little rifles, fun to shoot. They are getting to be quite collectible.