Old English is generally considered to have arisen post Roman conquest of what we now call England. It is largely an amalgam (my term) of quite a lot of other languages extant at the time and then it took a life of its own.

Prior to the Roman invasion (55,54BC and 43AD) and subsequent conquests, the predominant languages here were Brythonic or Brittanic. I believe that the last Cornish only speakers (a pair of sisters) passed away in the 1960s or 70s. The language has thankfully been revived somewhat and West Wales (Kernow) is rediscovering its roots. I also believe that shepherds across quite a lot of the modern UKoGB used to count their sheep in Cumbric numbers - the language of Cumbria (note how Cumbric looks suspiciously similar to Cwmru).

Discussions about Indo-European languages is paleontology for language! I'm not so good with me Greek to work out the correct term.

The notion of IE languages is analogous to fossils. We are talking about languages that are living, breathing and spoken. Obviously we must allow a bit of slack when ascribing certain attributes but I doubt any language scholar is going to get upset at Welsh being described as older than English.

The history of these islands has involved invasion after invasion after invasion. How on earth that history has resulted into only a few nationalistic identities and one empire is quite something. Bear in mind that sea travel is far quicker than land when you don't have roads so being islands means easy access and not hard access.