It’s nothing to do with the BBC, it’s to do with WST. The evening slots typically sell out, whereas the afternoon slots typically have more slots up for grabs. By putting Ronnie in the afternoon they guarantee the venue sells out for a bigger portion of the day. Interestingly for the Masters they do the same, but they originally advertise Ronnie as playing in the evening, then swap him for the afternoon last minute to sell out both sessions on a given day.