As you will see by the responses so far, it all depends on your attitude to risk!

You can afford to be there early but you cannot afford to be one minute late if you are battling the LHR T5 computer controlled conformance - whereas at another airport you can sometimes be allowed through by a human if they know the plane is running 20 mins late.

If you are travelling for leisure at your own time your risk category may also be different than if you are on a business flight paid for by your employer or client. I also prefer to err on the side of caution and hence spend a lot of time in the lounge.

Almost all my travel is for business and whilst I am content to explain my late arrival or non-arrival by IRROPS as that is out of my control, I would not be comfortable to say that I had missed the flight through my own fault.