Also, apologies for the double post but they replied to someone on Facebook in the comments saying before the end of the advent calendar they will give the "Day Splits and maybe we'll throw in something more" which makes me think they're hinting towards that GNR date being real. Could be a dodgy translation though.
Who are people expecting to see/would like on the undercard? Biffy Clyro, Amyl and The Sniffers, Flogging Molly, The War On Drugs, Wunderhorse and Frank Carter & The Sex Pistols all seem like realistic shouts to me.