First up, make no mistake, growing Cannabis, without a license, is a crime. Under The Misuse of Drugs act 1971 it is punishable by up to 14 years in prison.
That's the bad news.

Here's some good news.

Most people who are getting caught growing a small amount for personal use are getting a caution or a small fine.
Of course it depends on the exact details of your setup, where you live or whether the judge got his end away the night before.

(This bit is still in note form because I have been too slack to finish it. Rest assured though it will be done soon so please come back.)

Why Grow Cannabis ?

The cannabis you grow will be the best you will ever smoke.
Love, care and attention paid back.
Take cannabis out of the hands of major drug importers/criminal gangs etc.
Stop people moving on to hard drugs by not having to mingle with dealers.
You know what variety you are smoking, become connoisseurs.
You know what chemicals have been used egg pesticides, insecticides.
Stop exploiting third world farmers, dodgy regimes. Sick of buying shit hash or weed.
It's free almost..... (well cheap anyway).
medical reasons.
Taking responsibility for your own drug use.

Reasons Not To Grow

you might get caught and end up in prison.
It requires a certain level of commitment.
Paranoia.
You will start noticing helicopters everywhere.
People you live with may get stressed out.
You may well be caned for the rest of your life.