Artifact Signing (Trusted Signing) Public Trust identity validation for a UK sole trader: which document satisfies "government business registration"?

I am a sole trader in the United Kingdom applying for Artifact Signing (formerly Trusted Signing) Public Trust identity validation. I filed it as an Organization validation, because a UK sole trader has no Companies House registration, and the quickstart says Individual developer validation is only available in the United States and Canada.

The request is now in Action Required for the second time. Vetting Operations asks for "official business registration documentation from an official government agency that lists company name, address and contact information", and lists three acceptable types: formation documents (certificate of incorporation, partnership deed); a government-issued letter, licence, registration or certificate with dates and business information; or a record on a government registry website.

Already uploaded: a bank letter, a domain purchase invoice, and an HMRC tax-year document (uploaded under "Formation documents" because no closer category was offered). I have received three "Request for additional documents" notifications, so I believe the next upload is my last attempt and I want to get it right.

A UK sole trader is registered only with HMRC for Self Assessment. HMRC issues a Unique Taxpayer Reference (UTR) and periodic letters showing legal name and address. The organisation name and address on my request are my legal name and business address, and they match the documents already provided.

Questions:

1. For a UK sole trader, is an HMRC Self Assessment letter (legal name, address, UTR, dated within the last 12 months) accepted as the "Government issued letter, license, registration, or certificate"? If so, which Document type should it be uploaded under?
2. If not, which document can a UK sole trader supply?
3. If Public Trust organization validation is simply not available to a UK sole trader, please confirm, so I can plan an alternative.

I can share the identity validation ID privately.