That is because Japan is a major trading partner

I have to agree with Doug about the efforts of soldiers from across the Empire who helped in the fight, men from S.Africa, Rhodesia, India, heck pretty much everywhere.
Not many people know of the contributions and sacrifices made by the Indian troops for example (nowadays India and Pakistan). They fought with incredible courage in N.Africa and served with the highest standards of professionalism. Many troops came from places like the West Indies and also did incredibly brave things.
There were also non-Empire personnel who were superb as well, Ireland supplied many soldiers, sailors and airmen who fought with distinction.
Other surprising countries including Iceland (1 Spitfire pilot, Tony Jonsson read his excellent autobiography Dancing In The Skies, Grub Street), Denmark (Andres Lassen, the SASs only VC during WWII, there are two biographies on him one in Danish by his mother and the other in English by Mike Langley), and even Switzerland (Arturo Fanconi, who died on Utah beech trying to reach a trapped comrade after rescuing two. He had his feet blown off and bled to death, the only known Swiss to have fought in WWII).