Purely out of interest, was it a full alarm or intermittent in your area?

This link shows the fire assembly points for both terminals. Typically the area required to be evacuated would have continuous bells and an automated message (or at least it used to) and the safe areas had intermittent alarms.

LGW fire assembly points

I'm guessing you were in either the departure lounge or down Pier 2 when the evacuation commenced?