The article you quoted is must be very old. It refers to a recent reclassification of cannabis to a class-C drug in UK law, which happened in 2001 (and was reclassified as Class-B in 2009). It also refers to a BMJ article published "today", which I assume is Comparing cannabis with tobacco.. This article attempts to correlate the severity of cannabis smoking deaths with that of cigarette deaths. The basic argument they make is that cannabis and cigarette smoking have roughly equal risks of death, and that typical cannabis smokers "smoke less but inhale more", so they ought to be able to make estimates of cannabis mortality by studying cigarette mortality.
However, there is an article from around 2003, Comparing cannabis with tobacco - again, which appears to be a rebuttal of the original article; it's abstract begins:
Link between cannabis and mortality is still not established
A recent editorial in this journal implied that as many as 30 000 deaths in Britain every year might be caused by smoking cannabis.1
The first study done in a cohort of 45 450 male Swedish conscripts, age 18-20 when interviewed about the use of cannabis, reported no increase in the 15 year mortality associated with the use of cannabis
The second study was performed in a cohort of 65 171 men and women age 15-49, who were members of a large health maintenance organisation in California, United States. They completed a questionnaire assessing their use of cannabis, and reported no increase in mortality associated with use of cannabis
This second article seems to imply that the original BMJ article did not really establish a mortality level of cannabis, and that the 30 000 number was simply an "editorial opinion" based on cigarette deaths. Even the quotes from the publishers of the editorial in the Daily Mail show a strong level of weasel-wording:
In today's issue of the British Medical Journal, Prof Henry and other doctors from Imperial College, and St Mary's Hospital, both in London, say cannabis could be a major contributor to UK deaths.
Researchers calculate that if 120,000 deaths are caused among 13million smokers, the corresponding figure among 3.2million cannabis smokers would be 30,000.
Prof Henry added: "Even if the number of deaths turned out to be only a fraction of the 30,000 we believe possible, cannabis smoking would still be described as a major health hazard.
(all emphasis mine).