I'm not sure what ISO threading means on a headset. If "ISO" in this context is the same as "French," it is not compatible with British threading.

I've only ever dealt with British, Italian and French headset threading. My understanding is, and my experience has been, that British and Italian headset/steerer threading are ever-so-slightly different but not enough to matter; mix 'n' match your British and Italian headsets and forks to your heart's content. French headset and steerer threading is completely different and is not compatible with British or Italian threading and vice-versa.

As has been mentioned, you do have to pay attention to the crown race diameters, as those can and do vary by not a lot, but enough to be require some annoying filing or finding a lower press-in cup from a from a different headset,