I imagine you chose 80 years ago because of WW2 but it's not just since then. The UK has been an large importer of food for as long as food could be imported. It became an important issue during WW2 as supplies were severely curtailed as a result of the war (hence the "Dig for Victory" campaign).
The reasons for imports are many and various but here's a few:
- Many of the things we like cannot be grown here commercially (e.g. tea, coffee, cocoa, citrus fruits, pineapples, bananas)
- We like to eat many products all year round and even if they will grow here in the summer, they cannot be grown in the winter.
- It is often far cheaper to import food than produce it here.
- Farming is a shrinking industry in the UK. As a result we simply cannot produce the amount of food required here.
- Many foodstuffs are cheaper to produce abroad.
- The UK's population is growing at a faster rate than its food production capacity.