Wanting cannabis/marijuana to be legalized in the UK

Hello I am thinking about writing an email to my local MP because I would like for cannabis to finally be legalized in the UK for medical reasons as one of its benefits and also I would like to see it be legalized for other reasons I think and for the minimum age to smoke cannabis to be set at age 18 as we become adults at that age in this country for many other purposes so I don't see why it cannot be set at age 18 like most other countries have set it at that age and because of many other things having a minimum age of 18 too. Also we should all be allowed to make all types of decisions as adults who are age 18 or over like whether to smoke cannabis or not or do other stuff as no age restrictions should apply to adults either because age restrictions should only apply to children and teenagers who are underage. I really need some good advice please.