Guest, that has my head spinning a bit!
I'm not sure how I would approach finding out the percentage of the UK ( GB plus N.Ireland) population who supported/didn't support the monarchy.
Indeed, if a referendum was held to decide the future of the monarchy - and it was preceded with lengthy TV debates dicussing the pros and cons of a monarchy, I'm pretty sure that the 20% figure would double at least!
I see people's support for the monarchy mirroring people's belief in god; that is, they are born into a family that instills those beliefs into them.
Or, if you like, they simply keep the status quo going.