If everyone is relying on everyone else for products then nobody can hold anyone to ransom. Take Saudi Arabia for example - I think the largest Middle East exporter but I'm happy to be corrected if not.
They, obviously, rely on digging up and selling oil to run their economy. Where do they get the industrial valves, pumps, taps, pipes, etc? They don't make them at home. What about the computers that run it all or even tangential things like the AC units that allow all their foreign experts work in that ridiculous heat? The medical equipment that keeps those who own the oil alive, or even the expertise?
If they shut the world off, then the world shuts them off. It's a global market now, you shouldn't worry about UK food/power independence than you should Rotherham's food/energy independence.
The following article/paper (I'm sure you've read it before) was written nearly 60 years ago. Have a look and see how globalised production was then - it's been growing ever since and will only go one way.
http://www.econlib.org/library/Essays/rdPncl1.html