I didn't necessarily mean sloclo. I mean snow hampering the airport severely. Perhaps the fog example is the better one. If fog hits most airports it slows things down but it doesn't grind things to a halt and cause mass disruption to the domestic network.
LHR is severely hampered in ways which other airports are just not and in terms of flights, BA choose to cancel them, but they blame the weather. The weather is a contributing factor, but it is the fact that it is LHR that means they are unable to cope they they decided to operate from there.
It is their commercial decision to operate into LHR and to CANX the domestics (to strategically protect their long haul, much like U2's strategic use of its standby aircraft) yet they can still blame the weather.
If they had operated from any other London airport their operation would likely be able to continue but because it is LHR then they're pretty much screwed. That is their decision to operate from there. So to play devil's advocate then weather does not qualify for compensation under EU261. However If my airline operated from an airport that is not so slot restricted then there's a good chance my flight would have gotten away despite the fog. But because they operate from LHR I could put in a claim against them?
LGW/LTN/STN slow down under LVPs but they do not collapse in a heap like LHR does. BA could operate their flights from there so surely I have a claim against them as their decision to operate for, LHR has left them unable to deliver me to my destination in a reasonable timescale/not at all?