18hrs is a scheduled block time, not a flight time, up to 30 minutes taxying out at LHR, 10 minutes taxying in at PER, flight time now down to some 17.3 hours whilst, alas, airways don't necessarily follow great circle distances
Exactly, and let's not forget the possible speed restrictions both on departure and arrival. There are so many variables on a long sector that airlines will build in an adequate buffer to maximise their "on time" arrival stats. Ryanair do a good job of that on their relatively short routes. Just see how many eastbound USA-Europe flights arrive between 30 and 90 minutes ahead of schedule on a good day. Same goes for Asia-LAX.