The Chunnel is essential to carry the vast amount of passenger and freight traffic between here and the continent. When either the ferries or tunnel are out of action the other just can't cope. The traffic backs up to the other end of Kent in under 24hrs.
As the Chunnel is Private Property flooding is the best method. The owner won't be screwed out of their investment as usually happens over here in time of war.
Fill it to within 2 feet of the OHL and leave the power on......none shall pass.
:)
Could we some Aussie great white laser sharks just for sport?