It is over 100 years since Britain has been able to grow our own food. The population is just too large. Add to that the fact we can import food cheaper and it is understandable why we import so much. Britain has priced herself out of the market in many cases. We, the buying public, want to buy goods cheaply, the supermarkets want to sell to us, and if rubbish Granny Smiths are cheaper from Bulgaria than here (never mind what they taste like), they will buy them and we will be stuck with them. (Mind you, Granny Smiths originally came from Australia!)
It is the same with holidays. I often go away for a long weekend, somewhere in the UK, and I could have 10 days in the sun for a similar price (well I could before the £ fell).
As for the average age of farmers. The average age of the UK population is over 40 now and getting older. There are plenty young farmers, just their fathers are still living and owning the farms. We are not going to run out of farmers.
As an aside, the Scottish government is stepping in to buy up butter, to protect commercial bakers and the like. The reason there is a butter shortage is because farmers are shying away from milk production. And the reason for that is the fact the supermarkets have depressed the price of milk so low, farmers cannot make a living from producing it.
No one can blame a farmer for turning his land over for solar and wind turbines. They need to make a living and a return on their investment (the land) so if farming, per se, doesn't pay, then why not energy production.
It all stems from government policy since before the Great War.