Sounds like you have had some truly dreadful experiences, and as a childcare professional I think it's worth talking to ofsted about the preschools that have failed to include your son.
I do remember how it felt to despair of any preschool being able to keep my son safe. I had a couple of awful experiences too. I had pretty much decided that I was going to have to home educate him! However, eventually we did did find a lovely preschool where he was very happy.
The preschool allowed me to send in food for my son (he has a range of allergies which made him almost impossible to cater for, and we all felt safer if he only ate food from home). They went through all their junk modeling and craft stuff to make sure it was free of his allergens, and checked every baking activity with me well in advance. I sent in a treat box for when other children brought in birthday cake etc do that he could have a safe treat and not miss out. At snack time, ds was sat next to an adult, with an empty space the other side of him, so that he was on the same table as the other children but safe from cross contamination.
All staff had epipen training, even the Secretary. I felt confident that they would know what to do in a crisis, and also that they were doing everything they could avoid a crisis. Ds was happy there and he thrived, as they were lovely people who were kind to absolutely all of the children and always made him feel totally included.
I know that we were lucky... and that we have also been very lucky with his infant school, who are equally lovely and equally careful. However, finding allergy friendly provision is possible.
Of course, every preschool should be this good as a matter of course - but I do think you have to make a judgement call about whether or not you actually trust the staff and have faith in their procedures.
Worth mentioning that neithe ds1s preschool or infant school have made the whole premises allergen free - but they both made sure that allergens were contained at a table, they never let children run around randomly while holding or eating things ds is allergic to. All children have had their hands wiped after touching or eating allergens, and staff have made sure their clothes aren't covered in yogurt or banana or whatever.
Good luck with this - hope you find somewhere!!!