The largest supplier/only supplier (?) of weapons to the Assad regime is Russia (Two freighter loads every month). The AI weapon is described as being an "Arctic" Version of the rifle. These were not supplied to Assad by Accuracy International, a Portsmouth based UK Company, these weapons, or more correctly the particular weapon shown is most probably one of a consignment of these rifles that were sold to Russia:

"British-made Accuracy International Arctic Warfare Magnum (AWM) sniper rifles with elements of Syria's secretive special forces in Harasta, Damascus, new information indicates these British sniper rifles were most likely delivered to Syria from Russia.

While acquiring several AWM sniper rifles, albeit via the black market, would already prove a challenge for the Syrian military, acquiring these with the rare and very new pistol-grip skin would undoubtedly be beyond the capacities of the Army Supply Bureau of the High Command of the Syrian Arab Republic, which has so far limited its orders to Russian and Iranian sniper and anti-materiel rifles."