A fully managed end-to-end service for digitally signing code, documents, and applications. (formerly Trusted Signing)
One thing worth ruling out before you spend an attempt on documents. From the Quickstart:
"The billing account type must match the identity validation type: a billing account with an Account Type of 'Individual' can only be used for individual identity validation. You cannot use an individual billing account to validate an organization identity, or vice versa."
Worth confirming which type yours is set to, since a mismatch there can't be fixed with documents.
1. Is an HMRC Self Assessment letter accepted, and under which document type?
There's no published per-country document list, so whether an HMRC letter clears the bar is Vetting Operations call rather than something documented. Your recency is fine though: the FAQ requires documents issued within the previous 12 months.
2. If not, which document can a UK sole trader supply?
As mentioned in point 1 there's no published per-country document list. FAQ points that, which may serve you better: "If it isn't possible to provide additional documentation, update your account information to match any legal documents already provided or your official Company registration details." So ensure to align the account details to what HMRC already says about you.
3. Is organization validation available to a UK sole trader at all?
The UK is listed among the regions where Public Trust is available to organizations. Whether a sole trader counts as a "legal business entity" isn't stated anywhere I can find, so I can't confirm it.
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