Have there been any Green Party heads-of-state so far ?

[Nichlemn]

I'm almost completely certain there has neither been a Green HoS nor a HoG.
The Green candidate in Finland's last presidential election ended as runner up with 37% of the vote, but that's the closest anyone has come so far... well unless you count Prince Charles, he's technically been a heartbeat away from it for years, but it's not like Old Liz intend to die anytime soon.

I have always wondered about the Royal Families private political views. What proof is there that Charlie likes the Greens though?

The royals can't vote (or at least, they're strongly advised not to) and are not supposed to espouse political views for fear of compromising their neutrality.