We had a similar incident at Boscombe Down in the late '80s. An F3 with 2250l tanks was taking off on runway 05, the pilot raised the gear and we think that his index finger touched the Emergency Jettison button. The jettison buttons on the F3 were in a different position to the IDS/GR1 and no-one had anticipated such an inadvertent EJ selection. The buttons were subsequently redesigned to prevent this happening but I cannot now remember the details; I think that they were either recessed or a cover was fitted.
In the incident at Boscombe the fuel did not catch fire which almost caused more problems because the main runway was totally contaminated. I was running the squadron ops desk when it happened and it was quite a battle to get the airfield re-opened to get the aircraft on the deck and shut down within 2 hours of the incident such that the Accident Data Recorder was not overridden such that we could work out what had happened.