I don't think it's inevitable, and Wessex hadn't, in fact, conquered Cornwall (West Wales, not South) at the time of the Viking invasions.
I'm not saying we'd be smiley happy living in peace with the stereotypical Celts, but I do think there would have been less of a motivation to build an English empire in Britain, and also less centralisation. The Normans essentially wiped out the English ruling class and installed a new one, all from one source, which was centralised in London. Without that, you keep the looser Anglo-Saxon structure which would have had trouble uniting behind a concerted conquest of, say, Wales.