Same here! (UK)
I was under the impression that my national insurance number would serve as a tax ID, is this not the case?
That should be correct, and you can use the national insurance number as a foreign tax ID. Judging by the comment made below by leafo, sounds like the tax withholding rate might be calculating at the time you request a payout, so if you’ve provided that already, it might not show 0% instantly.