In autumn 1915 British 10th (Irish) Division had only three Field Artillery Brigades (LIV, LXVII, LXVIII) with 48 18pdr. field guns and no howitzer or mountain gun. In 1915 during the heavy fighting with Bulgarian Army near Kosturino, Bulgarians captured 8 field guns.
ASIK in 1916-18 Bulgarians had no chance to capture more British guns. I have no evidence of using guns British captured by Bulgarian Army.