as it is a requirment to have an instructor rating issued by the country you are flying in by the UK CAA to instruct in another country.
since when? Whilst it would be desirable, the CAA have no remit to insist on such a thing. The FAA might have something to say about it though.
There are CAA FIs conducting professional training in Florida with no FAA CFI rating and I have seen a letter issued by the FAA accepting it.