I dont think it's fair to say that the economy in the UK would crash as a result of stopping the sale of tobacco. From what little information I could find on the net in a casual search for some numbers, the information I found for America seemed to indicate that the money from taxing tobacco was spent mainly on campaigns to prevent youngsters starting to smoke, and also fed into the health service. I've no doubt that it would have an effect, and I can't be sure tobacco will be banned but it does seem like the next logical step now that the detrimental effects on users health is so well documented.
The cannabis research farm you are refering to appears to be GW Pharmaceuticals who are based in Salisbury, Wiltshire. They are currently researching the use of certain chemicals in cannabis for medicinal use. The problem with the pharmaceutical use of cannabis is that you cannot patent a plant, whereas you can patent a drug. By patenting a drug the company makes a profit by having exclusive rights to sell that drug, otherwise the research costs do not make business sense.
There are 60 known cannabinoids in cannabis (the psychoactive chemicals), but the two identified chemicals are delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD). The remaining 58 known cannabinoids in cannabis are still largely unresearched.
Cannabidiol is not intoxicating but it has anti-inflammatory, anti-convulsant, anti-psychotic, anti-oxidant, neuroprotective and immunomodulatory effects. GW pharmaceuticals currently have a product called Sativex which is an under the tongue spray and is mainly used as a pain killer for those suffering from multiple sclerosis.
One thing that did actually make me lol while researching Tetrahydrocannabinol was that one proposed lethal dosage would be to smoke 1500 pounds in 15 minutes
I don't think it's really fair to put cannabis in the same category as heroin and cocaine, but then I would argue that I consider the classification of drugs as more or less irrelevant. In some respects classifying a drug like cannabis as class C makes it seem harmless to an experimental teenager, and trying a class B drug then seems more harmless. It is the addictive qualities of the class A drugs that makes them more risky, but by that point the harm has already been done. So from that perspective you could say that there should only be one classification of drug.
References:
http://en.wikipedia.org/wiki/Cannabishttp://www.gwpharm.com/research_cannabinoids.asphttp://en.wikipedia.org/wiki/Cannabidiolhttp://en.wikipedia.org/wiki/Thc