It’s 20.6 miles wide at Dover. Its greatest width is 150 miles.
Gee, I wonder if the British still have those humungous coastal artillery naval rifles in serviceable condition that they used to use to trade rounds with the German guns at Cap Gris Nez. :-)
Edit: Nope, checked. The last ones were removed in 1956. A 14" naval rifle would be devastating to modern ships, though.