There are a whole range of bacteria which can cause food poisoning, all of which have different incubation times. By knowing which one was the cause, the OP can certainly provide a time line which shows that BA may have been the culprit.
It is actually good to know that BA are following up on this as they need to know if their suppliers are capable of meeting basic food hygiene levels to protect their passengers from getting sick. While they are at the mercy of their suppliers to some extent, they should also monitor complaints to make sure no patterns are emerging.