The Martini is a very strong action. It was originally designed by Peabody (an American) but that design had an outside hammer. Martini improved it by making the firing pin internal, with a coil spring, cocked automatically when the lever is operated. It is a very fast action (much better than the trapdoor Springfield of the same era) and served the British Empire well into the 20th century with .303 conversions using standard Cordite loads. I would have no concerns about strength with normal .303 loads in an ENGLISH made gun.
I am a bit surprised that they would claim Afghan made guns were English made. As Disciple says, it should be pretty easy to tell the difference.
Just a note on British nomenclature of that period, but I don't know the rationale for it. The first name was the inventor of the action - Martini, Lee, etc. The second name was the type of rifling used. So a Martini-Henry was a Martini action with a barrel using Henry rifling. A Lee-Metford was a Lee action using Metford rifling. A Lee-Enfield was a Lee action, with rifling developed at the RSAF, Enfield. They also at one point, included the information that the rifle had a magazine, and that it could be loaded from a charger or "stripper clip." The full nomenclature, with the word "rifle" implied as the first word, and commas inserted by me, would be something like "[Rifle,] Magazine, Lee-Enfield" or MLE. Another example is "[Rifle,] Charger Loading, Lee-Enfield" or CLLE. The full meaning of SMLE is "[Rifle,] Short, Magazine, Lee-Enfield." So it is the rifle that is short, not, as some folks thing, the magazine.
Jim