As far as UK laws are concerned, I'm an advocate of legalization of marijuana. It would do an immense amount of good.
Right now, in the UK, main problem with weed is that it's sold by and dealt by gangs and petty criminals, all illicit. Which is why it's so exceedingly easy for kids to get their hands on it, and it's also easy for those same kids to be pressured or introduced to far worse drugs which they did not intend to use, it's the whole effect of the environment weed is dealt in, it's filled with the entire market for drugs including worse drugs.
An idea might be to treat weed like we treat cigarettes. Tax them, regulate the supply, by regulate I mean; the actual chemical content and that it's only sold to adults who are responsible for their own health. It would also benefit by taking the gang/criminal element out of the equation, it would starve a lot of the informal market's funds, you'd also have less policing and costs associated with it.
Last of all, kids and adults alike will be smoking this drug no matter what kind of laws are in place to prevent it. The laws have failed. At least with legalization you get a lot of side benefits, and if you want to limit consumption, you do it in a conventional manner; tax, plain packaging, restrict supply, apply stringent regulation... and you go some way in keeping it away from kids.