The UK is not self-sufficient in wheat, and depending on harvests has to imports millions of tons.
"Enough wheat to give enough calories to everyone" only means that in an extreme situation and with very strict rationing nutritional needs could be met. Which is self-sufficiency at the limit only.
Indeed. In World War II Britain was dependant upon food imports even with rationing. After a period in which German U-boats happened to be unusually effective at destroying civilian transports, it got down to maybe a week's supply.
The government assumption was that - once the population realises they're going to be starved - they will revolt and insist on agreeing terms with Germany in exchange for food. Given what we later saw in Japan (which did starve its civilian population and they did not in fact rise up and force Japan to surrender) perhaps they were too pessimistic, we will never know because the next convoy got through.
"Enough wheat to give enough calories to everyone" only means that in an extreme situation and with very strict rationing nutritional needs could be met. Which is self-sufficiency at the limit only.