Just to be clear. With current methods the UK would need 0.4 acres of farmland per adult to provide all food.
Most of the land we have, that is undeveloped. Or not already used for farming. Is not usable as farm land.
So while it is a solvable issue. It would take seriose spending on new farming methods. And either a huge increase in food cost. Or a change in how we currently fund food.