Heathrow Airport, NATS, Airlines, ACL, Met Office etc operate a CAA agreed Demand versus Capacity system when there is a threat of severe weather disruption. The CAA has agreed that cancellations from that process (as they are in the wider best interests of all travellers) are exempt from EU261 compensation although duty of care applies and indeed the CAA send "spot checkers" to the terminals to police this.
A very sensible and pragmatic decision that avoids the temptation for airlines and airport to play chicken and hope for the best.