I don't just buy British in preference to foreign imports - I grow it! I have a miniscule garden (some people don't even have that), but I grow most of the veggies I need in summer. However, I live on my own and I don't have a full-time job and I can't grow what I need during the winter months.
Let's get something absolutely clear! The UK has not been self-sufficient in food for hundreds of years. Remember learning about the Corn Laws at school? We weren't anything like self-sufficient even during WW2 and would have starved if convoys hadn't brought us food from the rest of the world. Even though, British production did increase, the choice was extremely limited.
UK food self-sufficiency would require a revolution in farming practices.
However, that's not what's being proposed. It's been on the agenda for some time to turn post-Brexit UK into Singapore-on-Thames. Free marketeers want to stop all subsidies going to agriculture and only businesses which make a profit would survive. The most profitable businesses in the UK are services. There is no thought about what would happen to other businesses, such as agriculture, which at the moment isn't profitable. From an economic point of view, food production contributes little to GDP and there aren't that many people employed in food production, although there are many more employed in processing.
It's not difficult to see how the theoretical economists' minds are working. This is nothing to do with "Buying British", the environment, land use or the workers. It's to do with profit.