If you feel there wasn't bad weather at the time in question you can take the matter to CEDR - see the main EC261 thread for details about that. BA will submit more information at that point, including weather data. CEDR tend to put a fairly strong test on BA about the "all reasonable measures" aspect to EC261 so that may work in your favour. On the other hand if there really was bad weather at the time then BA may have a point.