I buy my meat, poultry and eggs from a local farmer too, but it costs more than a supermarket. I live on my own and only eat meat once or twice a week. I couldn't afford it, if I had a family to feed and I doubt if the country could sustain direct buying for the whole population.
I don't eat potatoes, pasta, pastry, bread or rice and rarely eat root vegetables. I avoid tinned and bottled veg, because they're packed full of sugar and salt, especially tomatoes.
Most of my carbs come from raw salad veg and brassicas and my protein from fish and eggs. It suits me, but I don't think a whole population could eat like that, especially if we had to rely on UK produced produce.
Unless the country invests heavily in renewable energy sources, I doubt very much that we can be self-sufficient. The UK's wealth is built on trade and adding value to raw materials. I really can't see us turning back the centuries in our lifetime.
The UK has developed sophisticated trading and financial skills. Those are our real strengths and are now possibly being thrown away...or...maybe not. There are rumours that the powers that be want to turn the UK into a turbo-charged Singapore-type economy, which is worrying.