1. Staying the obvious the “ snow” excuse at LHR/LGW sounds highly unlikely. I don’t think you need historical METARS, but you could probably dig up weather records off the likes of the weather underground or wunderground website.
2. Yes BA are short of pilots..but the solution has generally been to roster those at work through the roof, in terms of hours, rather than widespread cancellations.
however FWIW I’ll chuck this into the mix...
3. Marseille ATC (not the airprort, the whole centre) have been and are still engaged in a long running on/off dispute which has led to flight cancellations to destinations in south east France and northern Italy over the last few weeks.