Building these dams will be a lot easier than getting people to stop driving SUVs and pumping CO2 into the atmosphere.
https://www.epa.gov/ghgemissions/global-greenhouse-gas-emiss...
That would mean those countries GDP is estimated at 250 to 500 trillion pounds or 320 to 645 trillion dollars
France 2.6 trillion, england 2.75 trillion, Scotland 202 billion, Netherlands 826 billion. Or about 6.4 trillion dollars per Wikipedia.
Very roughly it looks like they should have said 0.1 or 10%.
10% of combinded GDP is very much not an easy investment to make!
https://en.wikipedia.org/wiki/Atlantropa
Edit: some grad student more recently (2014) proposed this as well: https://phys.org/news/2014-08-gibraltar-sea.html
An example is eg. dutch Delta Works: https://en.wikipedia.org/wiki/Delta_Works , which (among other things) reduced the total length of sea defenses in Zeeland by 700 km.