not sure but someone here will know, if its for personal use some police forces (not many though) wont even go after you at all EDIT:https://forum.grasscity.com/indoor-medical-marijua... ask here, people there will know.
if you have no previous you're almost guaranteed a caution, at most a few hundred £s fine, but there are no set guidelines, it will depend on the judge and whether his wife put out the night before and whether he ran out of milk this morning. With all the cuts the police are getting lately, many of them are having to prioritise and so they are unlikely to pursue you unless they get a complaint from someone. If you use a carbon filter, don't sell it, and don't tell anyone, they really don't have any way of finding out nor would they care. A guy got caught down your way a few weeks back, he had 6 small plants in a tent and he went to court and was fined a few hundred, and had to pay court fees, and a £85 victim surcharge. I was like, who the fuck was the victim? Very weird lol