The map above of a LHR-SFO is about as straight as you can get. However, regularly flying the route, there is considerable variance. My northern Canadian geography is hazy but, further east you generally fly south of Iceland but it's not too infrequent that you will fly directly over Iceland, and once we were north of Iceland. Likewise, further west, you generally enter the USA over Spokane but I have come down over Portland and also much further east, flying over SLC.