Well yes if someone is determined enough to put subs to sea I'm pretty sure they could shut the tunnel
A very good point. So, let me summarise. We are completely surrounded at sea, nothing can get through. The Channel tunnel is stuffed and modern airlift capability is completely incapable of alleviating the situation.
France, only 22 miles away, for some reason cannot be used as a source of supply - presumably because they have already been invaded and are occupied.
Sounds tough- but we have been there before.
Who is going to do this to us?