It turns out that Castille decided to settle this matter of Caux for me

they kicked Englands ass and decided to return most of their French holdings to France.
Later on I betrayed France by dishonouring their call to arms when they got beef with Iberian wedded Castille.
I expected Castille and allies would be able to kick France's ass and take their lands, but of course the blue behemoth got the best of them without even breaking a sweat.
So this was a pretty bad move and made matters quite a bit more complicated for me, as most of my routes of expansion then allied themselves with France (Kildare, Scotland, Münster).
To me it seems that dishonouring a Call to Arms is rarely a smart move, even if it means getting dragged in a tough war (which this one wouldn't even have been.)
I just got great power status, next up will be a war with Provence to get Ayrshire from Scotland without France being able to intervene, to establish a land route to England.
A naval invasion of England might be feasible in a few years, but will be tricky.
Consolidating the Low Countries & the British Islands should be pretty straightforward enough though, my only fear is that I will get dragged into a war between Castille - France.