SASless (#22),
Has put the finger spot on it. We were fortunate that Hitler (and Kaiser Bill before him) were both land animals, and could not see that Britain could be brought to its knees without the need for invasion or mass bombing. Simply cut our ocean supply lines - the "Western Approaches" - and we starve in a few months.
Kipling warned about this a century ago (in "Big Steamers"):
"The sweets that you suck, and the joints that you carve
Are brought to you daily by all us Big Steamers
And if anyone hinders our coming, you'll starve".
Churchill said it was his worst worry of the War (and in '41 we were in danger of losing it - tonnage was being sunk far faster than it could be replaced). The convoy system - together with Bletchley Park was our eventual salvation IMHO.
Could we defend our lifeline again today, with what's left of our Navy and maritime
anti -submarine Squadrons ?