Depends matey, on how much your growing.
But yes it is definately a jailable offence. Since blunket decriminalized it to class c, the time you can spend in prison from growing cannabis has risen from a maximum of 5 years to 15 yrs. Taking it into consideration that you only do half your time, (if you plead guilty that is). So you'd be looking at a max of 7 1/2.
I don't fancy that somehow.

But I'm guessing that if your only growing a couple of plants then you'd get off with personal use, and Maybe just get a fine or summat.