"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
AirPods and AirPower: Everything We Know
Apple UK Special Event: September 18th 2007
The invitation states "Mum is no longer the word" and invites media to a press conference on Tuesday, September 18th at 10am.
Macworld.co.uk speculates that this could be an announcement for the European iPhone launch or possibly UK iTunes content.
Apple had previously said that they plan on announcing details about the European iPhone launch by the end of this month.