The Sovereign Grant is more for the head of state functions- staff, building, hosting garden parties, state dinners, transportation of the monarch.
If the U.K. Government sends a Royal to a foreign country, the U.K. Government would pay for it. A realm pays for a visit from a royal, like the Cambridges in Canada. The specific charity may pay for the royal engagement like when William went to France to watch England in the Euros that would be billed to the FA or if Edward went to Italy for the DoE awards billed to DoE award I would image. Then there are the moneys from the 2 duchies to help out.
Thank you Skippy Boo