Agree about the letter from the doctor for the Epipen, I have never been asked to show it but I'm sure there would be a time...
If the allergy is particularly severe, you may like to check with the airline whether they will be serving peanuts on board. EasyJet, although a disaster in many aspects, is fantastic for nut allergies, you only need to tell the head stewardess as you board that you have a child with peanut allergy, and they will not sell peanuts/nuts on board and will make an anouncement asking fellow passengers not to take their own out. The only question they will ask is if you carry Epipens (guess that that is to confirm severity and therefore the need of the special measures).
Peanuts are not difficult to avoid in Spain as they are rarely part of savoury dishes so you may need to concentrate in checking bread, cakes and pastries, but bear in mind that in most countries out of Britain severe food allergies are quite rare, and sometimes it's difficult to be taken seriously. If you are in doubt, order something else.
DS have so many allergies that we carry most of the food he will need (not because it doesn't exist in the country but because we don't want to waste our precious time trying to find DS's food in a place we are unfamiliar with). We carry a lunchbox with safe snacks and fruit so, if we find ourselves without any apropiate option in a restaurant at least DS has something to munch on, but that's rarely the case as most restaurants are more than happy to do something simple on request (like grilled chicken with chips -pollo a la plancha con patatas) if you explain your child is so allergic.
If you are going to Spain, and don't speak Spanish, print the following in a card:
Mi hija padece una alergia severa al cacahuete (y todo aquello que tenga, podria sugerir algun platillo que no los contenga, por favor?
Muchas Gracias
(My DD is severely allergic to peanuts, could you please suggest a dish that doesn't contain them?)
When we are in Spain we keep to paellas, grilled meats, chips and plenty of fruit that we get from the markets. We take so much care with his nutrition at home that we relax during the holidays in the knowledge that having a few days of repetitive food is not going to cause permanent damage