Most of Zeeland, North and West Flanders, Flevoland, and large parts of Utrecht and Noord Holland are reclaimed from the sea, starting in the 17th century (they won 3 wars by partially turning back reclaiming efforts during battles). There was a point in history when Brussels had a seaport. Ghent was built as a seaport, now it's 50km inland. When Ghent became unusable, they built Brughes as a seaport. That one is now 15-20km inland. Without constant effort, these would simply be reclaimed by the sea, not in a hundred years, not in a thousand years, but in a month or two. The effort to prevent that has been a necessity for 2-3 centuries, and they obviously are not about to stop doing it.
So I would bet that these governments will continue to raise land (importing sand mostly from germany), dam rivers as required and control their elevation and coastal tides to compensate. Hell I've heard about a plan to put a dam in the channel, which would slow down the currents and make the land reclimation a lot easier and eventually create a continuous land area into the UK. The UK doesn't like this at all, but if national geographic is right, and it's either something like that, or lose London, I see that changing.