Imperial imposition of a common language has been tried - unsuccessfully - time and time again
The population of Great Britain is overwhelmingly of British or Irish/Scottish stock. The language spoken by the vast majority of those people is a Germanic language imposed on them by a relatively small number of roughly fifth century invaders.
I think that "British," "Irish," and "Scottish" is what they are today. The stocks that
created these groups were many and varied, the major ones being Celts (particularly Brytthonic Celts, for whom Britain is named), Picts in the north, Romans, Saxons, and, finally, a tremendous influx of Norsemen. The venerable Battle of Hastings, was, in fact, a battle between two Viking nobles, and Dublin was founded by Vikings about 1100 years ago. Much of Modern English comes from Old French, so I don't really see where there was ever a language imposed on the entire British Isles. In the Roman-to-Viking era, nobody - nobody - ever conquered the Picts, for example, and I don't doubt that Pictish can be heard in modern Scottish dialects, from where it has probably influenced Modern English. The same is true for the western reaches of Ireland.
