English- Not really a mix of Celt-Saxon-Angle-Viking-Roman-Norman and Pre-Celtic population. Genetic and linguistic evidence points to the fact that the majority of English (excluding those of Cornwall, Cumbria and Yorkshire) are Anglo-Saxons with most possessing pre-Celtic genes (although these genes are fairly ubiquitous across Neolithic Europe)

Now we are dealing with the BRITISH empire not the ENGLISH empire, the massive contribution made by Celts to the creation of the empire probably exceeds the contribution of the English. In particular massive numbers of soldiers and sailors were Irish, if anything we are dealing with a Celtic empire in which only the upper strata is English.