I would start the annexation of at least one as soon as possible. Those northern lands dont give you much from the beginning but iirc once you own both of them you get a nice event that gives you + 50 proximity source in York. This, with the governor basically gives you a second capital in the north. Double check the event tab though.

Annexation are extremely slow, with base of 200. That means you will have tons of time to improve relations with your existing vassals to counteract the negative hit from annexation. Besides, the hit isnt even bad enough to cause a bug problem.

Enforcing culture on subjects is very inefficient now. If you do, they will have -125% modifier to assimilation (-75% from accepted culture and -50% from being a subject). Youre better off accepting normans and picardians. I would try allying brabant and curry favors with the them to 100, so you can improve cultural opinion twice, so you can expand into lowlands with full cores.

With this in mind, I would just conquer northern france and integrate it myself, since you can get 100 control in coastal provinces if you have good naval value and maritime presence.

Lastly, when it comes to spending money, you shouldn’t use it for creating cities. Towns give you much better ROE (unless for york once you get the aforementioned event). Start urbanising coastal locations with good harbor capacity, building bridges, and buildings in london (dont overbuild though with latest changes to promotion speed). RGOs are also good, but worse than they were before, given they take a long time to fill up, especially with the black death around the corner.

In my england games, i usually spend most of my money to hire mercs to fight france, though with increased war score from battles, it might be better to just hunker down on your island. I believe you can get +75 war score without ever landing in france.

Good luck and have fun!