The only reason marijuana is being legalized in the USA is because many many people use it anyway, so legalizing it would bring a lot of money in sales taxes. It's not because of the medical benefits or harmless nature of it, there's simply more money to be made with it legal than illegal.
That's why there's no signs of legalization here for the foreseeable future. Popular opinion is opposed to it and very few people use it, so it's not profitable in the UK.
Money makes the world go round. Simples.