Right, so either the airport is running something that can break TLS, or snapchat is feeding a stream of private messages to the government with location information provided.
Almost certainly the second.
Probably a root certificate that you need to install to browse on the airport wifi.
I’m not saying this doesn’t happen but I can’t remember ever having been asked to install a root certificate when joining an airport wifi. And I am confident that this has never happened when I’ve flown out from Gatwick.
Perhaps the GCHQ have a backdoor in snapchat then, which is comforting
snapchat uses certificate pinning so i dont think this would work anyways