I have been trying to figure out how the discounting in different countries works by trial and error. This table shows the results for books from five of the big publishers that I have looked at in four countries (I've only looked at Science Fiction and Fantasy books).
Do promo codes work? Y=yes, N=no, S=some books not others, -=not sold in this country.
Code:
Publisher (SF/Fantasy) AU NZ UK US
Penguin Y Y S N
Tor (Macmillan) Y Y Y N
Pan Macmillan N Y Y Y
Orion (Hachette) N N N Y
Little Brown (Hachette) N N N -
Orbit (Hachette) - - - N
HarperCollins N Y N N
Random House Y Y S N
Transworld (Random House) Y Y N Y