The CAA has basically given BA an exemption from EU261 (and to all LHR airlines) if they have to pro activly cancel flights because of ATC restrictions (whether for weather or any other ATC reason)
There is a page on the CAA website where all such cancellations are listed because of this reasons and included all airlines.
I don't thunk this has been tested in court but they would rely heavily on the CAAs decision to exempt the flights.