The population density of the bits suitable for agricultural activity is extreme. Nearly all of Scotland, half of Wales and a fair bit of England is barely upland grazing suitable.
The efficiency of the actual farms is massive in terms of yield per acre. But the government keeps importing cheap labour that doesn't want to work, so the population is now completely out of control
During the war every scrap of land including grazing land for cattle and sheep, was turned over to grow food, hence the meat shortage. Pigs could be fed on scraps and chickens could eat insects, so they were in reasonable supply.
In those days it was North Atlantic halibut, haddock, cod and herring, also mackerel. That was when we had a national area to fish in before the EU let any man and his dog fish them to extinction. Coastal species like bream, sole, turbot and plaice were available, but expensive.
We were importing sheep from Australia and New Zealand and Beef from Argentina, fruit from South Africa, and bananas from the Caribbean. Until we joined the EU of course. No longer allowed.