You need to get another English of Aviation Certificate
Presumably you mean a demonstration of English language proficiency as required by FCL.055?
"....which means you need to be tested by a UK CAA approved Examiner"
How do you know that the OP is not already at Level 6? In any event, is it that only a UK examiner can assess ELP? (hint: the answer isn't "Yes")
In order to use the rating privilege,you will need to go through a UK TRTO
TRTOs do not exist any more. You should now be referring to an ATO.
A TRTO training completion certificate will be issued...
See comment above.