Given the following context:

U-153 is damaged during air attack in the Atlantic, and its crew abandon to escape Chlorine gas leakage due to broken battery cells. Her commanding officer is overcome by fumes before he can jettison the ship's papers. Aware of the intelligence windfall that this represents, the submarine is taken by a British prize crew to be examined and inspected (in much the same manner that befell the real German U-boat later renamed HMS Graph ).
It is not long before British intelligence suggest a new use for the submarine as a Trojan Horse. A picked crew of volunteers led by Commander Tarlton take the U-153 back to war, to intercept and disable a German Wolf-pack; in this they succeed, even sinking the Wolf-pack leader in their subsequent escape.
Her mission accomplished the U-153 is attacked and sunk by a British Frigate whose crew is oblivious to the submarine's mission or identity. Commander Tarlton orders his men to abandon ship, getting his crew off intact before she goes down. Their rescuers are astonished to learn that not only are the men they recover from the sea all British, but by attacking they have just sunk one of 'His Majesty's submarines…'

answer the following question:

What is U-153 at the time of its sinking? 

one of 'His Majesty's submarines