is it canon that kravitz was faking the english accent or did griffin just change his character voice?
Sort of both. Originally Kravitz just had a cockney accent, but then Griffin kept fudging up the accent so he just made it canon that he just do the accents for work.
Kravitz: [speaking in a London accent] Yeah, it’s-it can be tricky dying. Um, technically you’ve never checked into the Astral Plane. And therefore you’ve never escaped. So… I’ve, on that technicality, I’ve called off your bounty. But, I swear, if you die again, that is it. No more Mister Nice Death. That’s it. Kravitz is gonna come a-callin’.
Taako: So are you… when we listen for your voice echoing after us, should we listen for a Cockney voice, or kind of a distinguished gentleman voice?
Kravitz: [forcing a Cockney accent] All right! Yeah, I jump between the two at will, don’t I?
Taako: Oh no! The other head on your shoulder Zaphod Beeblebrox-style has begun addressing us!
Kravitz: Listen, I can jump between them at will! Check this out. [lowers voice, forces a bad Cornwall accent] Oi, now I got another accent! What’s this one even called? I'onno–
Merle: [mimicking him] Wot’s all this then?
Magnus: I think that’s still Cockney, but with a mouthful of marbles.
Kravitz: [forces a very bad Transylvanian accent] I’ve got to switch between different accents to trick my prey! [returns to a London accent] That’s not true. It’s just a fun little character trait I’ve got.
Merle: You sound like Harry “Snapper” Organs from Monty Python.
Kravitz: I can also do a bunch of fun Michael Winslow-style sound effects! [makes machine gun noises with his mouth] Um, now Lucas–
Kravitz: Well, Taako, this is uh… Taako, this is a pretty unconventional place to have an argument about sort of the fate of a whole, sort of small community, but… I do like wine, so… [suddenly stops using the accent that Kravitz has spoken with since his first appearance] I’m sorry, do you mind if I drop the accent, it’s like– it’s really really hard to keep it up, and when I’m not on the job it just feels weird doing it. Is that okay?