This is copypasta'd from my reddit account so sorry if some of this has been said already.
Right, done a bit of research on who it could be:
The requirements are: Never played Slam Dunk, last UK tour was headline arenas.
Bring Me The Horizon - Playing R1 big weekend the same day. These are my personal guess, they fit the bill and it would explain why they're okay to play so early.
A Day To Remember - God, I really hope it is them. It could potentially be them, their US tour doesn't start until June and the 10 years since Homesick would be such a brilliant selling point. Also just realized that the supports for this tour, Knocked Loose and Boston Manor, are both doing SD. However, they aren't scheduled to be in the country and it seems a long way to go for Slam Dunk.
21 Pilots and Busted are the other 2 that fit the bill, 21 pilots are in the exact position as BMTH. A bit controversial for Slam Dunk in terms of music genre, particularly 21 Pilots.
A couple of rule outs: People on Twitter were suggesting Breaking Benjamin. It ain't, they're in the US at a festival called 'Ticket to Rock' apparently.
Simple Creatures also mentioned and initially assumed, but unless they were lying about headlining arena tours (or being clever because Mark Hoppus and Alex Gaskarth *technically* have done with separate bands), it's not them.
Paramore also, but they've played Slam Dunk before. Not them.
Well, apparently I'm completely wrong and it's gonna be Breaking Benjamin. Going to leave this up so we can all laugh at the wasted research I just did.