Just deduction really - Chappell isn't an R&L exclusive, Sabrina isn't a BST exclusive, only place either wouldn't headline is Glastonbury.
Both of R&L and BST are the types to shout "festival exclusive" as loud as possible also
If she was *actually* headlining R&L, they wouldn't allow her to play Glastonbury.
She's subbing, they can pretend otherwise but we should be better than that.