Isn't Patrick Stewart also in that? Speaking of him, I'm honestly kind of shocked he's never been on DW.
He is, yes.
I can certainly imagine P-Stew in the original DW, at least, before 1987. Once he was cast as Picard, I can understand why it hasn’t happened. There’s his unavailability when working in the USA and possibly his salary (though he has done U.K. tv such as guest appearances in Extras or a leading role in The Eleventh Hour.
The real issue is likely to have been finding a role for someone who plays such an iconic role in another sci-fi franchise. I know that shows like Babylon 5 or Stargate made good use of Star Trek actors but I don’t think they ever had eg Shatner or Nimoy. I can kinda see how it might just be too glaring or distracting, like having, say, Mark Hamill turn up in TNG might have been.