As early PoD as you want, make Cornwall a 'nation' within the UK, like Scotland or Whales. It doesn't have to remain independent from the Union indefinitely, it just has to hold out long enough and be important enough on the European stage to be recognised as a 'different' country to England after being (if butterflies don't apply) overwhelmed by English hegemony. Cornwall does not have to remain confined to it's borders from OLT, as a larger Cornwall is more likely to stay independent for longer. Effectively, I'm asking for a stronger and longer lasting line of Cornish Kings at the very least.