Hi Anna,
Since this forum is devoted to Civil War flags you might have better luck trying a WW2 forum and there are many out there if you Google it. But to partly answer your question I assume you are speaking about the flag of the United Kingdom. It was very common to have a foreign nation's flag aboard a ship to use as a "courtesy" flag to fly when visiting foreign ports. This may be what you have or it is simply a UK flag made in the USA which you can still buy today. Please send photo if you can. My address is: emmalpass(at)yahoo(dot)com
Cheers,
Michael