London City Airport Fun Day
http://www.funday2010.com/
London City Airport : Funday 2009 (2009)
It's primarily organised for the local residents with proceeds going to charity (Richard House Children's Hospice).
GA attendance is invitation only and it used to be the case that one needed some sort of connection with the airport, though don't know if that's still the case. Certainly space is limited on site during the day so numbers of GA aircraft able to attend is also limited.
The airfield operates unlicensed for the event which is why single engine GA can fly in. The aerodrome license otherwise precludes such operations.