OG
//As a matter of interest, do we pay for visiting US leaders protection when they're here ? //
Yes we do. The money comes out of the Government Hospitality Fund which pays for all the arrangements for visiting dignitaries on Official or State visits. I have attended many such functions and Police protection comes from the same budget.
The key word is 'Visiting'. If the Ginger Whinger was visiting the States he would be entitled to Secret Service protection. As they have decided to reside there,despite his spouse asserting that she would never return to the USA whilst Donald Trump was POTUS, it will not be afforded to them at cost to the U.S. treasury.
Considering Madam has snubbed POTUS when he arrived here for his State visit and Harry has made disparaging remarks, to Russian hoaxers, about his new host I am surprised and they are lucky to get house room over there.