Ask them which runways, specifically, they are referring to. If none is LGW, or her destination, or the only possible alternate (unlikely to be only one possible alternate in Europe), the excuse falls flat on its face. You can always check what they tell you for truth. If they become obstructive, file an action for the compensation you think you are due online using the Small Claims procedure. It's not expensive. If they do provide the information and it does not support their excuse, file the claim. But do study the regulation first, in detail, to make sure you have a valid case. You could check out
Martin Lewis' advice.