Did you take a screenshot of the original offer when you made the purchase?

Also, did you receive the Avios and now have them reversed or is it that you haven't ever received the Avios and have now been informed that you won't get them?

If the latter there's not a lot you can do as you haven't actually lost anything yet. You still have the travel insurance. You could try to contact the BA estore via the chat option and provide them with proof of eligibility (if you have it).

Personally I don't buy insurance based on getting Avios. I just go for the best value and most comprehensive insurance for my cash.