Depends as a lot of people still take world war 2 rationing as an example thinking but as a country we'd dropped to less than 30% between the 2 world war, we're currently at 76% foods grown in the country and 61% overall. The government made a huge push post world war2 to make sure if a suspected third world war were to happen the UK would be more food secure. So the UK can support its 66m population for its daily calorie intake ( especially in cereals which is currently at 100%). So we can support ourselves but the variety would suddenly drop if we had no imports, which we will have. Food isn't even in our top 10 imports.