Peanut allergy is so serious and common enough that I can't imagine any event like that in the UK providing peanut butter among the general catering - too easy to have it contaminate something else. Okay for people to bring their own so long as it's in small enough packs that it won't spread everywhere.

(The commonest anaphylactic food allergens: dairy, peanuts, shellfish, eggs. People with anaphylactic reactions to dairy - the commonest one - generally live like hermits waiting to die, and eggs aren't all that commonly fatal except for children. Which leaves peanuts and shellfish as the ones you want to allow participants to avoid).