Presemably the cabin products are the same for services from both bases or perhaps the LHR 777's fly longer sectors than the LGW 777's do.
Not really.
Every 777 service from LHR leave with anything between 11 and 13 CC depending on configuration and not taking into consideration any load factor. It's the same at LGW as they either leave with 10 or 11 CC depending on configuration.
Many of the LGW trips go to the Caribbean and they can be pretty long.
The shortest trip for LHR is to TLV and even that one has 12 CC.