OK to be controversial - but I suspect I'm right - I think all the nightmare scenarios are totally OTT. I reckon the UK could adjust and be totally self sufficient in food production provided that we still had fuel available to allow efficient (which does not also mean gourmet or the most healthy) modern food production methods to continue.
Farming has improved in efficiency to a huge degree over the last century and I suspect that we are close to self sufficient as things are and off only a fraction of the land area that used to be under cultivation. OK we import lots of exotic products from around the world and can pretty well get anything we want with total disregard to the seasons - but the UK exports food too. If the borders closed, we'd have to adjust our diets but I doubt that many people need starve.
The key issue is whether this came about suddenly and there was no time to adapt or existing practices could not be maintained. In that case as they say any country is only 3 days away from a revolution.
If there was time to adapt, then we could increase the land area under glass - commercial greenhouses are very productive, polytunnels are quick to set up - etc etc and be sorted in a season or two.
Funny thing but as much as I love the outdoors / bushcraft skills, if civilisation got broke, technology is the very first thing I'd turn to to fix it.
Does that make me a heathen or just prove that (like Stuart) I was born with the engineering gene ?
):
Chins up ...