The maximum penalty for possession of cannabis is five years. Considering that dealing is a separate offence (and, I think, you get automatically upgraded to dealing if you're holding a certain amount - anyone know if this is true?), that means you can do five years in jail for carrying a single person's supply of cannabis. What's that - a half?
The maximum sentence for rape is life imprisonment - however, the average sentence seems to be about four years.
That means that, according to one person at least, holding a few spliffs' worth of pot is worse than the typical rape.
Great moral compass, eh?