Best to make it absolutely explicit - this event is not suitable for people with food allergies that cripple or kill them.
That sounds wholly and IMO unnecessarily negative.
I don't have any dietary requirements myself, so my views are necessarily secondhand - but given that my brother has coeliac disease and my wife has a string of food allergies (eggs, nuts, mushrooms, shellfish ...) that could see her whisked off to hospital, plus a wheat intolerance, I'm reasonably aware of a range of needs. (I also rode about a third of PBP with a coeliac rider, so saw how he coped there.)
I didn't see anything on LEL that would have prevented either my brother or my wife completing, had they been riding, and assuming a reasonable level of preparedness (including some iron rations in the saddlebag). They are perforce highly aware of anything that might be 'contaminated' and will ask on the merest suspicion: honest answers are very helpful - and not always forthcoming in professional establishments.
Realistically, a bit of basic forethought can make many dishes suitable for all (use GF stock cubes, use cornflour to thicken sauces), a bit of prep (have GF pasta and bread or rolls available on request) is useful, having ingredients lists available is very reassuring.
But let's not mix up the dangers of, for example, nut allergy, with the discomfort and other symptoms caused by sensitivity to gluten or coeliac disease. They probably need different guidance.
eg We cannot guarantee nut free food. We will attempt to provide a gluten-free choice at each control
Sounds about right. But what we can - or should - be able to guarantee is that *this* dish doesn't have any nuts or eggs or mushrooms or wheat in the ingredients, and that we've done our best to avoid cross-contamination. I'm pretty sure that my wife would see that as an acceptable risk, even at Eskdalemuir or Traquair, with an ambulance half an hour or an hour away.