That is not the current game though, and those titles have very significant game mechanical effects, and I seriously don't think gating those effects on papal opinion, even if only for catholics, is a good idea. If you change the system so all the benefits of being a king instead are granted once the realm passes a certain size, and all the benefits of being an empire are granted when passing an even larger size, and making it so anyone can vassalise anyone once they are sufficiently larger, than yes, the titles don't matter, so gating them on papal opinion is harmless. The game ties things like vassal options (a grand duke can't have a vassal king, even if he clearly has the ability to win the war before breakfast), and demense limits, to these effectively arbitrary and ahistorical title ranks. To say someone can't be an ingame King or Emperor, despite ruling half of Europe, because he is a catholic and the pope doesn't like him, is fairly silly.
The British Isles were a kingdom (ok, the full unification into one kingdom is WELL out of period, but three of the in game kingdoms had been mostly united in the game period).
Most of CK2s de jure empires were, historically, only Kings. What defines a King in game? What defines an Emperor in game? Are they deeply significant titles, or, in game, are they simply a way or representing the fact your are the biggest baddest nastiest person around, and have leveled up to being able to keep a larger class of vassal in line?