It is very complicated. The problem really is that the Royal Family are so massively integrated into the state itself. In some senses they _are_ the state (so how does the state tax itself?)
The French got it right, once they got rid of their Royals they knew they'd been gotten rid of, and they _stayed_ gotten rid of.
Ours came back and melded themselves into the heart of the state itself. Pretty disastrous, IMO, and I don't fully understand how that happened, but it seems, as far as I understand it, to be a consequence of the way the anti-Royalist fight got completely entangled with both a religious conflict (Catholics vs Protestants) and a war between the constituent nations of Britain (Scotland, Ireland and England, mostly). Somehow the complications of that mess let the Royal freeloaders sneak back in again.
In theory the money they get from the 'civil list' was supposed to be payback for the partial-appropriation of much of their assets, but they also got, in return for that, relief from having to pay for the full cost of running the country.