I was on LHY-PHX a few years back. Was going to grab a bite first, then head to airport. Got a dodgy feeling, and decided to eat when I got to LHR.

Was a fire in the tube, leaving me standing there close to 1 1/2 hours. People around me missing earlier flights.

finally get to the airport just at the 2 hour window, complimented the agent on how calmly she was handling all the testy people who missed flights. Lovely lady then decided I needed a seat change- to Biz.

Leave early- you can always eat, shop, read. Better without the stress.