It depends entirely on how bad the situation is. If it was a virus that killed a lot of people (75%+) then I would think it would be likely someone with knowledge of self sufficiency would be able to survive.
If it was nuclear then we are all buggered, for a start its unlikely you would know where the strikes were, there would be very little news and any infrastructure left would buckle within days, then there is the fallout afterwards, depending on the wind most of the UK would be covered, not to mention the strategic placement of our reactors means that if they were hit there is a nice big ring around the country of radioactive zones. A good film about this is an old BBC one, called Threads.
If it was the collapse of the social structure, financial collapse, food runs out in the shops, fuel runs, government failure etc out then no amount of survival skills would help you, you would be fighting the other millions of people for anything left, including any livestock. Having a "bag by the door" would be pointless. I very much doubt many of them would care about sustainability either. Any woodland we have left would be stripped within months for fires, any animals, wild or farmed would be killed for food, the country would be dead within a year. The best you could do is find a nice cave up in the highlands or something, you would still meet "competitors" not all friendly. People soon turn to their violent nature when they have mouths to feed.
We are prepared for a few days of disruption, be it electrical failure or weather, we carry in the cupboards two weeks worth of food most of the time, torches and candles are all in the same place, our house is old and draughty so we have plenty of warm blankets stashed.
Its nice to dream though and there is no harm in it, if anything its good for the mind. Problem solving and thinking of possible flaws in a plan helps in every day to day job.