Crusader Kings II

Assuming, like me, that you don't have any DLC of any kind which could potentially make this a faster ordeal, you are gonna want to start as William the Conqueror (specifically the one after he has conquered England. From there, I would recommend first annexing Wales, either by making false claims to the land or by other means. Since Wales is still split between Powys and Deheubarth, this should make it ever easier, as what little men Wales has is now split in half. This will also give you the bridge to enter Ireland.
Now you're left with Scotland, which is likely still in one piece, and Ireland, which is almost definitely divided into small independent rulers. In my opinion, taking out Ireland first is easier, for this reason; you can target small duchies and walk right over them as opposed to a full-on invasion of Scotland. Doing so will also give you plenty of time to reduce your threat level, avoiding other third-parties from getting involved.
Once most of Ireland has fallen, you will be able to create the title 'Kingdom of Ireland', which will allow you to simultaneously declare war on the rest of the island at once, which should be a walk in the park. (Make sure you have also created the title 'Kingdom of Wales' as well if you haven't yet, as it's part of the requirement.)
Finally, once Ireland is annexed, the last free Kingdom of Scotland is your last target. You now want to take over county by county via false claims etc, and then create the title 'Empire of Brittania' allowing you to take the rest in one grab (Assuming you win)