Wearing a symbol of a country on your person somewhere doesn't mean you are a citizen of that country. He probably mugged someone for it. By all means he looks the type who would do that. Or, like someone else said, he got it off a dumpster. Hey, maybe he even stole it from the Salvation Army?
Dante is not British. He's never had a British accent. I've never seen him as American either, even though his voice actors have all been American. He's a fictional character who speaks a language we all understand. He's not anything but that.
On a final note, why didn't they put the Underground symbol on him? So much cooler than the flag!