In the news

Mainly because it's against the law.

It's legal to grow it at home, either for your own consumption or for sale to others. But even if it's just for your own use you still have to pay the duty and your premises must meet the same standards as a cigarette manufacturer. Section 2.10 https://www.gov.uk/government/publications/excise-notice-476-tobacco-products-duty/excise-notice-476-tobacco-products-duty#liability-to-or-payment-of-duty

Nothing to stop you from starting a farm and setting yourself up as a fag company. The UK climate is suitable almost everywhere, so long as you harvest before the first frost. One frost will destroy the crop. This happened on the farm I worked on in Canada years ago, near Lake Erie. There were a few counties full of farms, with a few hundred farmers. We were six weeks into the harvest with another six weeks to go. (The plants mature from the bottom up, so you start by picking all the leaves at the bottom and come back a week later for the higher ones, and so on.) One night there was a frost. It was so unexpected that most farmers weren't insured against it. A few hired blowers and helicopters to shift the cold air. It didn't work. The farmer I worked for had no insurance so he threw us off the farm. (We were living in his barn.)

I doubt a UK farm could compete with countries where labour is cheap. Maybe you could invest squillions in robotic harvesting machinery. It's a very difficult plant for a machine to harvest because you mustn't touch the leaves which aren't ripe yet. The stems snap because they're very brittle. Picking by hand avoids wastage but it's so unpleasant that nobody wants the job. That's why I was there - European students were suckered into it on an exchange scheme. It was the worst job I ever had. A tractor on stilts drags you past the plants and you try to grab the ripe leaves and stack them in a bin. https://www.youtube.com/watch?v=L77jnl6xC20