Agreeing to do an interview is one thing. Being comfortable doing it is another. Not everyone is comfortable doing interviews, no matter how big a star they are. The One Show must be a weird experience, especially for guests from the States who are unfamiliar with the show and a lot of the stuff that is featured on it.
You make it sound like he's a one man band from hicksville, rather than an A-lister with an appropriate agent, publicist etc.
It's not like he's someone who's come 5th on a talent show and has to accept any TV appearance going.
He has the clout to say I ain't doing this or that.
Is he still with CAA? If he he is, they must not get on well if they thought the One Show was a good vehicle for him.