Let's add that:
1. Even if this whole idea worked 100%, creating a 100% killing zone within the range of the German coastal guns, it can work in the Dover Straits. The rest of the Channel is wider and the Royal Navy would have plenty of room outside the range of fixed coastal batteries. So the Germans have to attack on a narrow, narrow and very, very predictable front. No Blitzkriegs, no daring outflanking maneuvers. WWI on a pitifully short front around Dover. End of invasion.
2. Night. The Royal Navy's destroyers are specifically trained to fight night actions, and those coastal batteries aren't going to hit them at night.
So the Germans stop running supply convoys at night, this half chokes the landing force. So the British send in outdated battleships and monitors at night, and bombard the Dover and Folkestone ports, as well as the landing beaches, at point blank range. This chokes the remaining half of the supply lifeline of the landing force. End of invasion.
3. Smoke. Warships have this little trick to turn day into night. Sure, some of the destroyers might be hit anyway. So what, there's plenty of them and they are expendable. The outcome is the same as the first half of #2 above. Coupled with night actions as per #2 above, the supplies stop coming. End of invasion.
4. The funny thing about the Seelöwe scenarios is that it seems that the secret cool weapon that makes them work, only works for the Germans. Use sea mines, they'll kill the Royal Navy OK, but if that is true, why shouldn't the British also use them and kill the invasion armada? Same holds true for coastal guns. End of invasion.