I think England has been established within its present boundaries since the Norman Conquest- and for some time before it. My Saxon history is scrappy and I don't know exactly how and when the Anglo-Saxon kingdoms coalesced into a single England. Was that the work of Alfred the Great? I'll have to look it up.

Before then you have a period when the whole island was "Celtic" and parcelled out between warlords- kings, princes, whatever- and the modern nations had yet to emerge.

Until quite recently the Cornish had a language of their own- but I believe it has died out.