The trick with most London airport nowadays is to make sure you have your boarding pass scanned at the entrance to security no later than 30 mins before departure time. This is very strict and a 30 sec delay can result in a missed flight. Once past this hurdle though, it very much depends on whether the flight is on time/crowded, etc...

Out of experience with Easyjet, Ryanair, etc, if you make it to the gate around 15-20 mins before departure you will be fine.