I find that when flying to the US I can drink more or less all the way across (maybe a glass of wine per hour + an equivalent glass of water) and be fine and sober when I arrive there.

Coming back if I have more than 4 or five glasses – I arrive at LHR feeling like I have been out on an all day session, taken ecstasy, snorted dubious substances and been dragged through several hedges backwards and then forwards for good measure.