Self sufficiency is unachieveable because lots of foods British people want to buy can't be grown in this country like bananas, the most popular fruit in Britain. Being self sufficient means guaranteeing prices for home grown foods that will cost the taxpayers a lot of money and there will be a lot of waste like the EU had in the 1980s with wine lakes and butter mountains.
Britain also can't grow tea, coffee, chocolate, pepper, etc that British people really like. Britain tried to limit cheap food imports once with the Corn Laws but it only lead to high food prices that benefited land owners at the expense of ordinary people and was eventually repealed.