Yeah I've been banking on L+R having LP for a while now. Easy double payday, hype name, 2000s rock throwback they always have, will get ~30 year olds who can afford pints in the arena.
From memory, the Park event was £60-70 this year. Slightly dearer than the City event, but some bigger acts, and of course, guaranteed entry, no queueing at venues.
Kneecap's tour is pretty big and sold out - I'd say they're definitely co-headline size now. The film has made them pretty big and there's a lot of talk for an Oscar nomination for the international category, which would make them even bigger before the fest.
I think it'd work and they'd be a great booking.