Unfortunately I can't see this working, if it was recent, since BA can legitimately say that the weather / ATC delayed the arrival into LGW. We have had a few days of pretty bad weather - well, bad from an aircraft perspective, if it was just a bit of rain it couldn't cause this much disruption. I think you have seen elsewhere that the root cause here was that the previous LGW-MCO service was delayed due to weather on the flight before that, so 2 flights before your flight: there is an argument that BA should have more spare aircraft for routine English summer weather, so at a pinch you may be able to claim on the lack of "all reasonable measures". But because of all the storms I'm not convinced that his would work, even at CEDR.