A UK RT examiner can assess non-native speakers during the RT practical, but only at level 6 (expert level). If the examiner thinks your English does not meet the level 6 requirements they cannot sign you off at a lower level.
Ps: Native English speakers get their English assessed as well. I am not a native English speaker myself but as a UK examiner I have assessed native speakers to be at level 6. Don't ask..........
PPs: Your instructor is not authorised to sign off anything when it comes to language assessment. Only examiners can do so.