This is a British Military combat shirt in Desert DPM (Disruptive Pattern Material). This pattern was used by the British Military in arid and desert combat environments until the recent adoption of the MPT (Multi-Terrain Pattern) camo, which is similar to Multicam.
The combat shirts feature a breathable mesh body as well as camo sleeves and shoulders. The shoulders are padded for comfortable use with body armor. Also, the sleeves have pockets with hook and loop fields. You can adjust the padded cuffs with buttons. There is a small British Union Jack flag sewn onto the left shoulder.