Hire a few times before you decide on anything. Ask to have a feel of other people's guns, most people are more than happy to let you have a look at their stuff as long as you ask nicely. Once you've had a chance to handle what each of the gun types feels like to hold make a decision based on which was most comfortable for you, some people love AKs and some people just can't get on with them, same for SA80, M4, G36 etc...
As an under 18 you cannot become UKARA registered or buy a gun, two tone or RIF. Your parents or another adult will have to buy it for you, there's no flex on that.