I feel like she'd look more like a headliner in the UK if other UK festivals would deign to book her as one, so far it's just been stand-alones and boardmasters. Like she's headlined most fests in the USA and everywhere in Europe and continues to do so, it's just the UK where she hasn't rly been booked.
She's been selling out arena tours of that size for the last few eras with no issue, I think it's her lack of other UK fest appearances that cause others to doubt her cred in that respect. Tho she is definitely 'smaller' than GnR/arctics/Elton.
Her last album is a monster live, would go over well at glasto if given the chance imo