WWI and WWII destroyed Britain, they didn’t win squat. Lost their Empire and most of their manufacturing base. Lost so much manpower they had to import labor from the colonies.
Germany (twice) and Japan (once) got crushed much harder in every way than Britain. World War I had an enormous demoralizing effect on Britain, especially on the upper classes, who died at a much higher rate. World War II was similar in that relatively speaking, it had a much greater effect on British national pride and self-image than on Japan or Germany’s. It was humiliating for Britain to see itself playing second fiddle to America, a country almost entirely without imperial ambition, and they deeply resented it.
What they mainly lost was not just fighting men, but men with fight. It is tragic.