The idea
The two main reasons given for the illegality of marijuana is that firstly it may cause cancer, and secondly it may lead to mental illness. Neither of these claims have any significant proof behind them. Look at the evidence, there is no firm proof that cannabis is harmful like alcohol and tobacco, however I will try and avoid pointing this out as cannabis should not be made legal just because it is less bad then the two most lethal drugs out there. Instead I will try and make a logical and irrefutable argument against the prohibition of marijuana.
Marijuana is a plant and THC is a drug inside the plant. If the government were justified in making marijuana illegal then surely the more THC within the plant the more harm the user would come to from smoking marijuana however that simply is not true. A hash (concentrated extract of marijuana) smoker has to smoke less to achieve their high so the risk of cancer coming about from burning side products of the plant are reduced. So by making marijuana illegal you have essentially made it more harmful to the users health, turned it from something which is VERY unlikely to cause cancer, and in fact has been shown to have a curative effect on some cancers (inhaling a small amount of hash smoke) to something that may or may not cause cancer (smoking herbal cannabis).
Furthermore prohibition hasn't prevented cannabis use, it has merely pushed the profits in to the hands of the dealers at great expense of the tax payer. Dealers are now cutting their cannabis with fibreglass beads in order to add weight, while nobody really knows the negative side effects of smoking fibreglass it cannot be good for you. This is a huge problem effecting nearly all herbal "Skunk" sold in the UK. This is getting ridiculous, prohibition is stopping nobody but it is harming everybody's health. If THC were truly a harmful drug then hash would be worse for you then herbal cannabis not better for you. As their doesn't seem to be any reason to keep cannabis illegal why keep going?
Why is it important?
Look at the Dutch model, legalising cannabis greatly benefited the economy, it reduced hard drugs use, and their country isn't falling apart at the seams as government circulated propaganda implies legal cannabis will do. Any law which prevents somebody from an action which only harms themselves is of questionable morality, any law which causes people to come to harm because of what normally wouldn't be harmful (in the case of spray) is objectively and unquestionably immoral, I understand that after demonising weed for such a long time its hard to say "we were wrong, its not as bad as we previously thought" but surely the time has come to explore whether keeping marijuana illegal is really benefiting anybody at all, chances are if you are reading this you have smoked marijuana once in your life at least. How would you feel if you then got arrested and charged for possession which would effect your later life more then smoking cannabis, when the punishment doesn't fit the crime the law must be changed, keeping cannabis illegal is an attack on the liberty of UK citizens.