That's what history used to tell us, but recent science tells us that it is not so. DNA from ancient burials in England are identical to those of modern inhanbitants. We are, on the whole, descendants of the stone/bronze age inhabitants of these isles.
In the 18th and 19th centuries, the less powerful nations of Europe sought to increase their social cohesion through nationalism. Especially after the French showed how this could be used to raise and motivate giant armies (hardly any of Napoleon's infantry spoke French, at the time that language was practically unknown outside Paris by any except the rich).
So countries like Poland and Slovakia, which really didn't exist, created national languages, costumes, foods and myths. The Scots and Welsh joined in because they had something to distance themselves from: Britain, which was usually referred to as England at the time. England didn't join in in the same way, because we had all the money and the power. In effect, what those other countries were doing, was trying to recreate the accidents that made England so much richer and more influential than they.
This modern desire to rewind to the 19th century and join in with the flurry of nation inventing that went on abroad is really just a symptom of a national loss of confidence. Those things are the trappings of infant nations, and not only should we not need them now, but nor should any established nation. Making a special day for saint George (a Greek), or making a special national anthem (probably one written by a German), seems a pretty shallow way to go about celebrating the legacy of a people who never previously felt so insecure as to need such things.
