According to Sheldon they have a slightly different spec:

ISO 1.375" x 24 tpi
English 1.370" x 24 tpi


Although strangely enough he lists them as the exact same here:


http://www.sheldonbrown.com/gloss_bo-z.html#british
http://www.sheldonbrown.com/gloss_i-k.html#iso


Anyway I have always heard that you can change them up, but don't keep switching back and forth.