Format
Ten digits — e.g. 1234567890. Sometimes shown with an “A” or “K” suffix in HMRC correspondence; the suffix is not part of the UTR itself.
Who has one
Any UK tax resident with a Self-Assessment obligation. A UK founder of a US LLC usually already has a UTR because the US LLC’s profits (if the LLC is treated as transparent) flow to the owner’s Self-Assessment.
If a UK founder somehow doesn’t have one, they request one from HMRC via the Self-Assessment registration flow. It takes a couple of weeks.
Where it goes on Form 5472
In the owner section, as the foreign tax identification number. The IRS accepts UTR as the foreign TIN substitute for UK founders.
National Insurance number
Not a substitute for UTR on 5472. The NI number is for the UK payroll/benefits system; UTR is the tax reference HMRC and the IRS both expect in cross-border contexts.