That is quite easy to do as it will happen right at the beginning of the game. As described, start at the "High Middle Ages" (1066) and pick William the Bastard as your character ( it should be in the "featured" characters). The achievement is quite easy as you already have around 11000 soldiers in an already raised levy at your disposal.
Now, the good thing is, Norway is also trying to conquer England at the same time, so you should wait for them to reach shore first, and if you're lucky as I was, the king of England will also immediately try to attack you, wasting his troops. If you don't move your army from your home province, you should be able to neutralize England's army quickly that way.
When you're done with them, sail to England and hunt and crush Norway's army, which should be divided in groups of around 3000 soldiers, then try to occupy any province you can. Once you reach 100% war score with England, enforce your demands and you should be named King of England, and thus win this achievement!