You can grow cannabis in the UK, however it can be a smelly crop which tends to draw unnecessary attention. If people are going to grow it outside, they tend to find a spot well off the beaten track, and only go back to harvest it.
The ideal way to grow it is indoors, where you can control the lighting (cannabis uses photoperiod to trigger flowering), temperature, humidity, and protect from pests. Whack a fan and carbon filter on, and no one will know anything.
People who grow their own cannabis do so as they don't much like mixing with drug dealers, and also they can be sure of what they are using (street drugs are notoriously contaminated with goodness knows what). Saving money is less of a driver, and unless someone is growing an awful lot, unlikely to happen, when you add up the cost of kit and electricity.
Oh and we already do grow cannabis in this country - as a cash crop. It's called "hemp" and you can buy the oil in your supermarket, and the seeds in your pet and angling shop. The fibres are used for paper, cloth, and rope. Being fast growing, hemp is a useful plant for cleaning contaminated earth. It also fixes nitrogen into the soil, so helps reduce the need for artificial fertilizer.