FRA is the likely contenders as Parisien pointed out. But BA doesnt fly the 777s anywhere in Europe, do they?
Defintly not Gatwick - only GA, CO , NW fly there. UL, QF fly to Heathrow.
And as Garuda do not fly to many points in Europe(only LGW, FRA and AMS) that rules most of these airports out.
Definetly not LAX - UL, GA do not fly there.
Definetly not SIN - CO do not fly there.
I reckon it could be FRA if the BA 777 was a last minute substitute, which could be likely.
Someone give us a picture please!