Just because City Hall is public doesn't mean that there aren't rules that you have to follow when you're there, or that you can't be kicked out when you break those rules. What it means is that you can't make up rules, because those are just the whims of the ruler.

If there are no rules at a publicly-owned cemetery against verbally abusing your recently dead mother in front of her grandchildren, you have to face that the abuser, as part of the public, has as much a right to be there as you do. But in real life there will be posted rules, just like there are rules at the public pool.

If Britain wants to pass a blanket law criminalizing all criticism of the royal family, they should. Plenty of dictatorships and monarchies already have lèse majesté laws, they can just copy them. They won't have to make up rules or improvise.