Hello Northanstar and welcome to FT!
I totally share your concerns but wouldn't worry too much! If you check out Zol's & Eightblack's fantastic Fleet thread here you can see basically all 2-class 777-300ERs have either been refurished or already sport the new lie flats anyway. LGW usually gets these if it is a 2 class aircraft (42J/386Y instead of 42J/400Y). I am always more concerned if it is a 777-300ER in three class config as there are still a few birds out there with the awkward old seats. All 777-300s sport lie flats by now! (they are sometimes on the LGW flights sporting 12 Skycruisers in F and 42 Lie flats in J)