At least until very recently, marijuana (or cannabis, as they'd say over there) was a very low priority drug for law enforcement in Britain. You could pretty much toke up smack on the top of Primrose Hill in the middle of London at noon and even if a bobby caught you, the most he'd probably do is get you to put it out, hand him the rest, and tell you not to do that. Or so I hear...
I think they upped the severity of it in the last year or so though, so things may have changed.
