There are lots of ifs and buts with allergies.
People with allergies sometimes have only had fairly mild reactions before then suddenly have a severe reaction.
People might have planned to bring a meal, but had it confiscated at security (we've had this happen to us), or their flights might be very delayed meaning that they'd ended up eating all the food they'd brought with them (again, happened to us).
People, for whatever reason, sometimes end up having to buy food at the airport, then get on a plane. Or they end up having to eat food they buy on a plane.
Food resellers, should do their best to label their food correctly. They might always be able to get it quite right for various reasons, but they should at least try.
In this situation, Pret clearly could have chosen to do a much better job of labelling its food, and chose not to do so.
A combination of factors meant that a young girl died. Some of those factors didn't have to happen.
Pret might not be legally liable, but some of this is down to them choosing not to label food in a way that they really should have bothered to do so if they took food allergies seriously.
I hope food retailers are watching this and learning from it that they do have a moral responsibility.