How do you know what guests will appear on the Graham Norton Show?

So I want to apply for some tickets to the Graham Norton Show, and there's many dates to choose from, but how do I know what guests will appear on the show beforehand? Often I hear audience members tell they knew in advance that their favorite actor would appear on the show and they got their tickets in advance, so I wonder, how do they know?

I love the show, but sometimes there's actors, singers or comedians I do not know, and since I do not live in the UK I feel it would be a little bit of a waste to watch a show with people I do not know.

Can anyone help me? Thanks so much!