Only revalidation via a skills test proficiency check can be accomplished by an examiner with a non-UK issued examiner authorization.
However, that alone does not entitle the non-UK examiner to endorse a UK licence. To endorse a Certificate of Revalidation within a UK licence, the examiner is required to have received specific authorisation. The UK CAA usually provide a one-off authorisation to do so along with the e-mail designating them for the proficiency check. See EASA Examiner Differences document and
IN 2016-003 para 6.
ifitaint...