I agree with most of the posters that 2hrs is typically adequate but I've had the following scenario more than once.
* late inbound flight that is the equip for my flight to LHR
* take off 25 minutes late...
* get to LHR airspace and take a few additional turns around the track, lose another 15 minutes (remember the OP gave a flight to LHR on a Friday afternoon)
* so get to the gate 40 minutes
* sitting in coach so 10 minutes to deplane
* gate is 800km from from the bus so 5-10 minute brisk walk
* long queue for the bus 10-15 min
* now I'm sweating because of the walk and my 2hr margin is rapidly evaporating
* bus arrives, long queue for security....my queue held up by amateurs
* another 800km walk (actually running now) to gate for my flight
* get there to hear "would passenger CVO 1K report for immediate boarding.."
Have not missed a flight yet but it's been close several times