I feel very strongly that we should be as self sufficient as possible, whether in steel, coal, nuclear power,transport or food.
We're a country of 65m. We need to produce at least some tangible or intangible exports, to buy critical resources, unless you want us to turn into North Korea.
How much of the country would you need to grub up, and how much of the population would you need to kill, for the UK to be self-sufficient for food, even at starvation rations? We didn't manage it, not even close, in the second world war. People who are nostalgic for deep-mined coal (ie, pneumosilicosis) by and large didn't work down mines, and people who actually did moved heaven and earth to make sure their children didn't: making the UK self-sufficient for steel would involve restarting deep coal production at 1920s levels: who's signing up for that work?
In order to export anything, indeed to produce anything even vaguely competitive on the world market, we'd need a microelectronics industry at least comparable to the US's, which isn't theirs anyway (they outsource manufacturing to Mexico, Malaysia and the Philippines, design work to Israel, Cambridge and India as a minimum, etc, etc). We'd need to produce software which allowed our companies to be competitive, so we'd need to compete with Microsoft, Oracle, SAP, etc. And so on.
This was the nonsense of the 1970s. UK companies were forced, by exchange control pressure, to buy ICL computers in order to "back Britain". But the result was that companies that could have been successful against US competition weren't able to get the computers they needed, and instead had to wrestle with expensive and inferior equipment. Ditto in the 1980s with microprocessors (Transputers, FFS), until that was all quietly lain in its grave.
This country would be self-sufficient if it were an agrarian country with a population of ten million, tops. For anything remotely like modern life, with anything even close to current population levels, we need to import. That means we need to export. That means we have to be competitive. That means we need to do stuff as well as other people, or better.