In my head canon they would probably get the dirt from the Amazon and Mississipi and buy black soil from Ukraine and Russia to put on top of it. Also i thought about fillinf in Zeeland but it already kinda is and it might disturb coastal communities and ports there. Also about other lands the southern tip is slightly farther from the English coast than Calais is.I feel this is going to be a lot easier for the English to swim too than Calais. A bit hard to tell without other landmasses shown. I look forward to a sequel that fills in the areas between the islands and peninsulas of Zeeland. Ahhh, but really. Imagine the Dutch politicians, trying to tell the public they are making Doggerland and trying to explain where they got the dirt from and if it won’t end up raising the sea level outside the Dutch dikes,