I mean, with two spots left, I could buy Stevie and Mariah as the two remaining headliners.
I also know you can get Thursday shows at BST - my stumbling block for Stevie is that the only weekend dates left at Hyde Park would be a fortnight later. Sure, he and the band could just kick their heels in the big smoke for a few weeks, but it doesn't feel like it lines up.
Somebody like The 1975, on the other hand, are all British-based (I think?), so it's not exactly a logistics stretch for them.