The simplest explanation is someone in the Snapchat group tipped the police and provided screenshots.
> A court in Madrid heard it was assumed the message triggered alarm bells after being picked up via Gatwick's Wi-Fi network.
The article states that the Spanish courts assumed his message was intercepted in the airport, not over the in-flight wifi.
It’s complete speculation anyway. As others have pointed out, the likely route the message took to the British intelligence services is via Snapchat disclosing it directly, not through some edge-network magic TLS-breaking packet sniffer.
...
They don't actually state that was how the message was acquired. Snapchat uses TLS to deliver messages, so how would it be "picked up" via Wi-Fi?