Well, yes. The position of the tunnel is well known. It should not be difficult to lay sufficient explosive on the sea bed, focussed downwards over the (relatively soft) chalk, to blow the concrete lined roof in. It's not exactly a U-boat pen, and we managed that (Tallboys ?)
But they wouldn't have to bother. The whole of Europe can't feed itself; the original idea of the Common Agricultural Policy was to make Europe self-sufficient in foodstuffs. Of course it produced butter mountains and wine lakes, but that's rather a restricted diet. We are reliant on Canadian wheat to make the bread for the butter (I believe the wine was mostly distilled into industrial alcohol).
That it became the boondoggle of all time is not the fault of the Founding Fathers. The idea was good, but......
D. (Armchair Warrior par excellance).