"Those selling food should most definitely know what ingredients are in it"
Sorry, but that is never going to happen. As NJ says, consumers will always have to take the responsibility for these matters on their own shoulders.
Loose food products packed for sale on site don't require disclosure of possible allergens contained within. They have never done so, but more importantly, have proved and are continuing to prove, a major problem for the FSA and other regulatory bodies in the UK and Europe, when it comes to deciding how to provide allergen information to consumers.
One would think it should be simple matter of having a list of potential allergens behind the counter or on a wall etc for the consumer to view and come to a decision. It doesn't work. Ingredients change seasonally, lists get removed accidentally or deliberately, staff are not as au fait as they should be on allergens, seasonal staff are indifferent on the matter and a whole host of other reasons. That's even before we take into account the pressure shop staff are under on a busy day.
Last Sunday, I bought a suet crusted pork pie at an indoor/outdoor market in Carew, Pembrokeshire, one of my favourite outdoor markets. I know the stallholder well, but I can assure you, listing all the possible allergens in meat pies over and above the legislated 14 required by the FSA is next to impossible. What would happen if the stallholder provide me with the list of 14 on a tick-box format and a 15th allergen that I was sensitive to, made me ill? Gelatine, a major ingredient in pork pies, has been known to cause an allergic reaction, but is not on that list of 14. Moreover, add a dash of Worcester Sauce to the pie and at a stroke, a whole host of other possible allergens contained within the sauce come into play.
As a scientist, I can assure you that I can think of at least twenty compounds in that pork pie that could have resulted in an allergic reaction in me, none of which are in that list of 14.
Simplistic methods of addressing the inclusion of potential allergens in products as undertaken by Pret etc are admirable and will go a long way towards preventing further tragedies. Requesting information from staff is always a good idea. However, these will never be the answer entirely: potential cross contamination in small, confined kitchen areas during preparation alone will see to that.