I can see why the short range aircraft have more crew due to the short sector times and a high club load but I cannot see why the LGW 777 needs less crew than the LHR 777 unless the LHR 777 has more seats or a different configuration that requires more crew.
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.