No one knows for sure. He lived at the time of the Roman Empire extension into Brittania (400s AD) and there are claims by England, Scotland and Wales for his original hometown which isn't well known. In any event, a patron saint doesn't need to be born in the country. The patron saint of the U.S. is Mary and in Canada its St Joseph.
Gotcha. I have seen that before. If he was born around 385 in the British Isles somewhere, as they claim, he and just about everyone else there were a least a little Roman anyway. The Roman withdrawal wasn't until about 410.