Latin did not supplant the Celtic-based languages of Britain beyond the Romano-British ruling class. It's very difficult to evaluate the extent to which Latin penetrated everyday use because the modern English language is by and large based on the Germanic Anglo-Saxon language, not some Celto-Latin hybrid. Indeed, when Anglo-Saxon languages became dominant in England there were enough holdovers of Celtic grammar in Old English to justify an entire theory that explains these linguistic quirks by postulating that the English language has a Celtic "Substrate." Foremost of the proponents of this theory is linguist John McWhorter; the gist of the argument is that when the mostly Celtic population of England gradually adopted Anglo-Saxon from Germanic migrants, some syntax carried over into Old English.