Sorry but I cannot accept that, presumably the AEW 2 is still in production and is still being shipped to retailers so how can they not have a supply of spares for that kit? To the RHS of this very page there is an ad saying "Avro Shackleton AEW.2 In Stock Now!"
Don't forget this is a manufacturing defect, I haven't broken or lost a part.
What you can, or can not, accept is neither here nor there. The UK's "Sale of Goods Act" places responsibility for replacing defective goods with the retailer, not the manufacturer.
It's to Airfix's credit that they are prepared to at least attempt to replace missing or defective parts directly.