I originally intended to have my current character Ælfric to first inherit Wessex before pressing his claim on the Duchy of Mercia, which would've led to the exact same result in terms of forming England.
However, things came up. I had plenty of time to press my claim on Mercia; all I had to do was bide my time because it was a weak claim. Doesn't take that much to be able to press it, simply either forcing the target to go into a regency or attacking during another claimants war will do.
Wessex had primogeniture from the beginning of the game, Mercia had gavelkind. There are certain advantages to gavelkind succession, mainly no penalties for not landing adult sons....however, there IS a fairly significant drawback: my various titles under my direct control would split up on the death of my current ruler, assuming that there are multiple children. (One at the moment, which presents an issue because it's a daughter)
My preferred succession, if not the law upon game start, is locked behind a certain law which only opens up once I upgrade my Legalism tech to 3. At the pace I'm acquiring the relevant points to dump into it (there's three branches of tech to upgrade from, each with their own points pool: military, economy, and cultural; Legalism is a subsection of cultural), I'm looking at holding onto the throne for roughly 40 more years. By then, I should have a good enough opinion bonus for a long reign to keep most of my vassals at count or duke tier happy enough with me to let me change back to primo. (No direct vassals at count or duke tier can have a negative opinion of you to do this, add kings once you reach the empire level)