Started as the earl of Oriel, united Ireland, made slow inroads into Scotland & Wales by pressing ducal claims, took a while but managed to usurp/form the two kingdoms and subsequently hoovered up the remaining independent states.
England was tougher, I found someone with a claim on the throne, got him to defect to me and arranged a matrilineal marrage between him and one of my daughters. They had a son who inherited the claim to the throne of England (after I killed his dad). Took a long time, but I managed to get him his kingdom. Then I had to make him next in line to my existing kingdoms. The only workable succession method was
elective, but I was working under gavelkind, so I had to change all my kingdoms' succession laws. This pissed everyone off, and no one wanted to vote for him except me! If I had died at that moment it would've been curtains, so I took drastic measures.
Revoking a bunch of dutchies enabled me to dictate the successor of each kingdom, although fighting off the annoyed nobles while waiting to die was difficult. Luckily one of them was so annoyed that they poisoned me. My heir, the king of England, inherited the kingdoms of Scotland, Ireland and Wales!
After quickly moving my capital back to Oriel I have spent the last 15 years in pathetic appeasement to my seriously pissed off vassals. My kingdom is intact and consolidating. Next up, France.