A part of me thinks weirdly they might hold off on Chaka/Robbie until they have a year where nobody else who fits the bill is available. This slot takes a very specific set of criteria, and I can't imagine they're flushed with options who are on tour and would take the smaller payday every year. Both Chaka and Robbie have expressed interest in doing it, so they're bankers for a year where fewer options are available. Which may well be 2027.

Chaka and Robbie got to have decent chances due to previous reports

Chaka is a west holts act all day long and yes please (as part of my vaguely purple day that I am still trying to make happen).   Teatime? Bryan Adams. 

https://www.kerrang.com/biffy-clyro-interview-finsbury-park-simon-neil-ben-james-johnson-futique   Interesting chat at the end there about the Biffy Finsbury Park show.