In the UK, you will usually get a small fine, unless you have previous.
However, they
could give you prison time in theory, and it is more likely that they will if they can prove intent to supply. In other words, ditch any scales and weed bags, and you will be a bit safer if the worst happens.
Check the legal and news sections of uk420 (
click here), for details of busts and cannabis related legal stuff in the UK.
There does seem to be a bit of a "postcode lottery" going on in the UK, with punishments varying wildly from one area to the next, and dependent on things like whether the judge got laid recently etc.
I wouldn't worry too much though. Just keep things low-key, don't say anything to anybody, control odours, and don't get into trouble for any reason other than growing.