Misuse of Drugs Act 1975, s 12
It’s illegal to knowingly allow your home or your car to be used for a drug offence (like letting friends grow some cannabis plants in your flat), even if you don’t have anything to do with the drug yourself.
The penalties depend on what type of drug it is. Allowing your place to be used for the sale or use of cannabis or some other Class C drug has a maximum penalty of three years jail; if they’re class B, it’s a maximum of seven years’ jail, and if they’re Class A, the maximum is 10 years.
However, you wouldn’t normally get a jail sentence for this offence unless you already had a history of minor drug offending.