Jack Dee is a very well known face on British TV, in part for hosting Live at the Apollo before it became rotating hosts. Grumpy, dry and deadpan, he's someone Alex has consistently mentioned when asked who he'd love to have on the show.
Rosie Jones has been on a lot of panel shows lately, and is quite a marmite taste - people seem to love or hate her. In my house we're not a fan in large part because her punchlines are all shock humour, which doesn't really work once you're expecting it. I'm hoping she'll grow as a comedian - Sarah Millican's routine seemed to be 99% I'm fat earlier on but now she's great.
I've not watched anything with Andy Zaltzman in and I don't know the other two at all.