It's easier to get the throne of France as the king of England, tbh. Invasion CB is pretty busted, since piety is so easy to get, and you only need to be smaller than the target.
It's fairly straightforward for William the Conqueror to gain the Kingdom of France at the age of 60 to 70. Good luck surviving the succession crisis, though.
Walkthrough:
1. Declare a Holy War for a random muslim coastal duchy in Iberia. Win. Create two bishops for 50 piety, grant them landed titles for the counties they are in for another 200 piety, and grant one of them the ducal title for another 200 piety. Get the "become pious" achievement for another 100 piety.
2. Wait for a French civil war and/or Regency while saving up money for mercs. Get an invasion CB, and declare war. Buy a couple stacks of mercs along with some vassal troops to go around assaulting counties for warscore.