We cannot currently grow enough food to support the population. And there is nothing remarkable about that. We have been importing wheat from Canada since the 1860s and the great farming slump that lasted from the 1870s to the start of WW2 was caused by the fact that our farmers could not compete on cost with prairie farmers elsewhere.
We still needed to import food during WW2 -and the population then was around 40-45 million, compared with 65-70 million now.
I am not sure whether the UK could be self sufficient, but if we were the diet, would be dull, limited. The medieval peasant subsisted on a diet of rye bread, pottage, a stew of beans, peas and onions, and probably kept a pig and occasionally added a fatty pork to their diet, plus foraging. Milk and dairy products were probably market products sold to the better off.
The industrial worker in the early 19th relied on bread for his main sustenance with cheese, occasional meat and amything they could raise on an allottment, if they had one.
Even allowing for the improvement in the range of cultivars and their higher cropping rates - and farming technology. Our diet would be very limited.