Who was NOT the final boss.

In most regular playthroughs of Sburb, the final boss is implied to be the Black King. Now, of course we know this isn't the case in both of the kids' sessions. Jack kills the Black King in the beta universe while HIC takes over the alpha universe and serves as the Derse royalty, which fits her well considering her blood color.

Lord English is just Caliborn, who is just another player. All he did was get himself stuck in the game for much too long and played it until the end of his life.

The beta kids don't have to worry about their final boss as their session was destroyed, but the alpha kids only had to defeat HIC to beat the game. If you recall, the house they enter does not appear until she is defeated. The door is only added once the forge was lit. The game was beaten. Lord English was just another player that got too strong and played around in the wrong parts of the game.