The 777s at LGW were not hand-me-downs though, so I don't see why a replacement fleet would necessarily be? I would be fairly confident about the long-term prospects of long haul at Gatwick, it was relaunched fairly early in the pandemic and continues to fly. The 777-200ERs will need to be phased out over the next decade, the A350-900 is the probably the closest direct replacement so I would not bet against it. The 777-8 is closer in capacity, but its enormous range apparently makes it heavy, the 777-9 is larger again than the 777-300, so probably too much capacity.
The 788s seem to have a great place opening up new routes from LHR to the US and the -9s were useful in Asia, at least historically. So I dont think hand-me-downs will be forthcoming. Move the A380s round the M25?
