Hard to say precisely. The first recorded mention was of marijuana consumption in the UK occurred at a Royal Society meeting in 1689. Robert Hooke, a Royal Society member and noteworthy scientist spoke of his experiences with the plant. His friend, a sea captain Robert Knox, had brought some for him as a gift after a voyage to India. Hooke observed it's properties firsthand and concluded that there may be beneficial properties.

Hooke's mention of cannabis was detailed on Neil DeGrasse Tyson's, "The Cosmos" series.

It's likely that Hooke and Knox weren't the first Englishmen to partake on British soil though. Wouldn't be far fetched to assume sailors traveling to/from India and/or the New World would have encountered the sticky icky.