Actually geographically false and culturally true. Geographically both Japan and the British isles are connected to the continent near by. It is the relative isolation which made for a cultural difference. Only the UK and Ireland use a Westminster style of government. The Euros are all republicans. Japan also uses a Westminster style of government.
But it's not only a matter of political constitutions. Britain is part of the culture of European Christendom, and has been for more than a thousand years. And Japan is part of the China-Korea-Japan culture complex, cultures all deeply shaped by Buddhism and Confucianism.