Last estimate I heard was 0.4 acres per person to grow enough food. And honestly, the UK does not have enough agriculturally suitable land to support its population.

Only way we could do it now. Would be to build multi level indoor farms. Technically, this is possible. But economically even more likely to require non-self efficiency. IE, groups be the societal or capital motivated. Getting together to fund, build and manage such complex and experimental farming.

Not to mention energy requirements. Multi level lighting may be possible using sun reflection. But the more levels you need, the harder and more likely you are to need artificial lighting. So this increases our need to consider division of land usage for farming vs solar and wind generation.

Honestly, I am not sure being fully self-sufficient. Will ever be possible again for dense population nations like the UK. We have not been so food wise since sometime before the 1900s.