The route back to London was up the west coast of Italy. My question is: who decides a flight's routing? The airline or ATC? Does it vary because the air traffic and weather varies? My return flight was a week later, but might the immediate return flight follow a different route?
At my operation (UK Scheduled) the flight planning department decides on the route.
There are various parameters that dictates their choice however the main criteria is Minimum Fuel Track (Taking into account headwinds and distance), Minimum Cost Track (which also takes account of airways charges) and avoiding congested airspace which is causing slot delays.
Sometimes they also have to plan around airspace which has been NOTAM'd as restricted or closed.
Some airways are weekend only as they are within military areas Mon - Fri.
The slot delay issue can be a big one especially when there's strike action within a country. For example, when the French controllers go on strike you can often be routed to the likes of Malaga via Lands End and then down to Santiago as this can avoid a slot delay of many hours.
SW