UK sole trader trading with US client - NI or UTR on W8 form?

Hello, I'm about to start trading as a sole trader in the UK with clients based in the US. I have recently registered for self-assessment, but have not received my UTR yet. In the meantime, my US client needs me to fill in the W-8BEN form, where I need to provide a foreign tax identifier. My understanding is that both the NINO and my UTR should be accepted, but as a sole trader it would be more appropriate to use my UTR. Since I have not received my UTR yet (and by the HMRC time checker my registration for self-assessment won't be complete until the end of December), can I use my NINO on this form?

Hi,
Yes, you can.
Thank you.