I'd doubt the 777-3s are being touched.
Suspect it'll be a conversion of the 777-2ERs G-YMMG-P (12/48/32/127) to 3 class (ties up with reduction of F seats and simplification of fleet) to align with the rest of the 3 class 777-2ERs 3 class G-YMMA-F & G-YMMR-U (some of which are already at LGW) and they'll all become 10 abreast down the back.
Then the LGW 777-2 nonERs which are 3 class will have their enhancement G-VIIO/P/R/T/U.
Either way, 25 is either too high a number or too small a number.