"Guilty" by the band Blue was co-written by a "The X Factor UK" judge that is a lead singer for what pop group?
Guilty (Blue song):  It was co-written by Gary Barlow, singer-songwriter from the popular boyband Take That. Gary Barlow:  He is frontman and lead vocalist of British pop-group Take That. Gary Barlow:  Barlow also served as head judge of "The X Factor UK" from 2011 to 2013 and "Let It Shine" in 2017.
Take That