I said, ages ago, that Simon would fight tooth and nail to get Adele on XF. However, she's already picked the BBC over ITV by having her special on tomorrow night on BBC1. So, pretty much anything she showcased on XF would have already been trumped by being on the Graham Norton interview/performance show. They may well get her on the finale, probably by ensuring that one (or both) of Louisa and Lauren are in the final so she can pick a young girl, as she once wanted to duet with Ella Henderson.
But Adele marches to her own tune - she'll only go on if she can actually be bothered doing promo stuff for her album. It's not like it's not going to be the biggest album of the year, even if she does zero promo for it. Plus, chaotic live show - always a far easier choice to do prerecorded stuff, like Strictly's results show - although XF have sometimes done a prerecorded performance to broadcast on the Sunday night show, so that's an option. However, I can see just as easily see Adele going 'go on then, quite like Grimmy and it's just for a laff init!!!'