I used to work in a job that required being in supermarkets for long periods of time. You could always spot the "growers" coming in for the supplies of potting mix etc. Some parts of the country have the ideal climate for the growing of the happy baccy and every summer the police helicopters would be out spotting the plants from above. Once spotted they were no longer a source of income for the owners.

My ex brother-in-law used to grow his own. He sourced his seeds from budgie food I believe.

I was having a quiet drink the other evening, there were two lads out on the pub patio having a smoke. They walked through the pub as they left; good God the smell of cannabis followed them, and didn't half linger.

At one time I did some freelance NVQ assessing. I usually only got the contracts because I'd work early evenings. Not naming the factory (but it was one of Coventry's biggest!) the place reeked of cannabis. The whole workforce was high on the stuff. Mind you, they did have mind-numbing jobs.

No doubt there are plenty more to keep the supplies going.
http://www.nuneaton-news.co.uk/News/Cannabis-factory-found-after-police-raid-20130516160947.htm