I got a similar problem with my bus ratings they are not yet endorsed on my JAR ATPL.
I believe you got to options; If possible convert your UK ATPL into a JAR ATPL and then go to a JAR TRTO i.e. Emirates Training Center and get a check on 777 (make sure the TRTO fills in the UK CAA check forms!).
An other possibility would be to get your MEP (Multi Engine Piston) Class Rating renewed either on the UK or JAR licence.
Good luck with the CAA
