What I need to do is to obtain the flightplans for a certain operator's routes from the UK to a couple of specific longhaul destinations. Are they pretty much constant, or do they vary significantly on a day-to-day basis eg because of weather?
Longhaul routes vary depending on weather and other things like airspace closures and there is also a lot more that is taken into account.
Routes out of the USA to Europe are in the public domain on some websites.
Are you going to be running a scare story saying xxx airline flies over Iraq and Afganistan lots on the way to the far east?!?!