All their income goes to the state in exchange for the annual grant (or something along those lines, I don't really know the details).
Ahhhh turns out I had that wrong
But I like they make voluntary "tax" payments on private income (less any private income spent for official purposes), when they're not obliged to.
What happens was actually included in his short accession speech yesterday (full text here https://www.theguardian.com/uk-news...rles-declaration-to-accession-council-in-full )
"I take this opportunity to confirm my willingness and intention to continue the tradition of surrendering the hereditary revenues, including the crown estate, to my government for the benefit of all, in return for the sovereign grant, which supports my official duties as head of state and head of nation."