LGW offered them a deal to move which makes sense as they've been close to insolvent. Assuming they own their LHR slots, that's money in the bank they REALLY REALLY need if they can sell them on. Air Mauritius is really struggling and has been bailed out by the government (again). Objectively they'll likely do well commercially at LGW, I'd suspect that the next management team will want them back at LHR, which may well be a bad idea for them. Politics seems to dictate things more so than data.
One thought though, every time this happens, people cry that this time it will be different, this time LGW will be back in long haul as a player. And almost every time, it's really just a waiting room for LHR slots. . There's not a long list of successful moves LHR->LGW, Pan Am, TWA, Malev......
BTW Air China isn't even bookable for a service starting in two weeks, what's going on here? Even LHR is only being sold via FRA and Lufthansa in April, the only LHR offering seems to be PVG. I guess someone is using their LHR slots for the summer? Surely they didn't lose them?
Air China 2019W for comparison :
Flt No ETA From Via To Season 1 2 3 4 5 6 7
CA847 06:20 PVG LGW N 2019W 2 3 4 6
CA787 07:10 PEK LHR T2 2019W 1 3 5
CA855 15:55 PEK LHR T2 2019W 1 2 3 4 5 6 7
CA937 17:50 PEK LHR T2 2019W 1 2 3 4 5 6 7
CA423 20:10 CTU LHR T2 2019W 2 4 6