Like everyone else, I'm in two minds about this.
First. they seem to be talking about Blake plus six criminals: that brings it back towards Terry Nation's original concept, of Blake plus the seven convicts from the London: Jenna, Avon, Vila, Arco, Gan, Selman and Klein. Wonder if they're creating new characters, or going back to Nation''s original ideas? Arco, at least, had a lot going for him as a character (junior defence minister, involved in a fraud set up by the Secretary, took all the blame when it went wrong. In other words, Wolfowitz, set up to take the blame by Rumsfeld, as it were).
Anyway, on the downside: Blake's 7 is just another space opera unless you get the interplay among the crew, and the Orwellian politics underlying it. Difficult to see SyFi going for the latter.
On the other hand... Martin Campbell has a major role. Edge of Darkness! Casino Royale! (oh, and the Edge of Darkness film, but everyone makes mistakes). If they'd been making the original series on film, he'd have been directing it, instead he was doing Minder and the Professionals for Euston.
And casting wise, I find myself thinking about people he's worked with before...
Assuming the past Sy-Fi pattern of some British actors who want to crack the American market (like Robert Carylyle in SG: Universe), some film stars whose star has fallen (Like Lou Dimaond Phillips), and maybe some pilot episode guests who'll bring in publicity and seem to be leads but get killed off rapidly (like Robert Patrick).
On that basis, thinking about people Campbell's used in the past and might ring... Ian McNeice, Ray Winstone, Joe Don Baker, Martin Shaw?
For Servalan, pick from Samantha Bond, Zoe Wanamaker and Joanne Whalley.
For short-lived publicity grabbing cameos, an outside bet would be Pierce Brosnan, Judi Dench or Mel Gibson... possibly as the Federation President. Though really, that's a part for Paul Darrow.