I will see if I can find you some specific information later and translate from French if need be.
I am often taking the Eurostar (since it is easy than flying most of the time - time and cost)
There is an M&S Simplyfood (Marks and Spencer food store) in St Pancras right across from the check in, and while it is a somewhat smaller version of their food halls it may be a good place to pick up some 'last minute' items as the labels will of course be in English. You may want to ask about food allergies on the UK community board as there are certainly posters there who can give you advice about which foods to purchase in England. They do a very good job of labelling and even mark things as safe for vegetarians and safe for vegans, which makes it easy for me.
Pret a Manger (a London/NY chain)
http://www.pret.com/ does great fresh takeaway prepackaged food and it is labelled, and they are also at St Pancras. So is PAUL, the great French bakery chain, but I suspect that they may not have much if they are using the same flour they use in France. I don't honestly know but wouldn't risk it. If you go to
www.paul.fr you can contact them via English language as they are in the UK as well, and I suppose ask if their French locations use lupin flour. I don't know if I would chance it but up to you.
There is also a Starbucks and a McDonalds in the Village (like Downtown Disney) so they may have more specific ingredient listings if all else fails. But the menus do vary by country so they won't necessarily have the same products and ingredients as in the American outlets.
Ben and Jerry's ice cream is in the parks, but Haagen Dazs has a location in the village at the entrance to the movie theatre. They have different flavours than in America, and I don't know if a sorbet would work? But that is something to think about if you are already familiar with alternative treats for ice cream.
The EU does have requirements as to certain allergens being listed, but if the list is in French, or the item is not packaged, it isn't as much help to you.
I also recall a question about food allergies over on flyertalk.com on the France board so I can take a quick look later and see if it has anything.
If you google lupin+France+allergies you may find some information in English (I seem to recall a BBC page from a few years ago) Whenever that question comes up I don't suggest items from the bakeries as they are usually not packaged, and as the lupin flour is added during the production phase to the wheat flour it may not be listed even if someone did take the time to go and check for you. Unfortunately that means avoiding some of the more tasty treats, but with egg allergies as well that would be out anyways.
Here is something as well to read, not that much but still some good information
http://chowhound.chow.com/topics/624177
And this one talks about where to order the 'allergy cards' in French
http://chowhound.chow.com/topics/620749