Reasonable excuse: domicile outside the United Kingdom
You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.
In death cases where the deceased was domiciled outside the UK the Probate Registry requires a certified copy of the foreign grant to be submitted before they will grant probate for the UK assets taxable in this country. You should not normally accept an excuse that the submission of the account was delayed because they could not deliver the account until the foreign grant or its equivalent had been obtained. Delivery of an IHT400 in respect of UK assets alone can be made prior to and separately from an application for a UK grant.