Presuming BA are claiming 'extraordinary circumstances' as their get-out clause (which unfortunately is not properly defined under the regulation) however i would note the following (taken from europa.eu);

9. In the event of cancellation of a flight due to ‘force majeure’, how can I check that it is indeed an extraordinary circumstance which could not have been avoided even if all reasonable measures had been taken?

According to the Regulation, the company has to prove that it is indeed due to such an exceptional circumstance. The passenger can always require the proof. If the passenger is not satisfied with this information, he can refer the matter to the body designated by the Member State. One aim of this Regulation is to also give a means of redress to passengers faced with arguments that airlines sometimes use to justify cancellations.

and

12. What can I do if the airline refuses me my rights?

You should address a complaint to the national authorities designated by each Member State. For the names and the addresses of these, you can call the free-phone number: 00800 67891011, or send an e-mail to the following address: [email protected].
The list of the authorities is also available on Internet at:
http://ec.europa.eu/transport/air/ri..._bodies_en.pdf .