I guess that depend on what happens for the next couple of centuries. How deeply does Roman civilization seep into the isles? When do they become Christian? Do they? Is there a polity extant when Barbarians invade that is strong enough to beat them back? I tend to think it unlikely that Germanic invasions can be completely kept out, but maybe they could be prevented from becoming overwhelming, and like the Norman invasions of France, able to assimilate newcomers. That could leave you with a totally different world, in which the British Isles speak a Romance language. Somewhat insulated from invasions, you might see British polities that are much more important in European history.