I might be wrong, but I believe that the only baking brand that does a nut free promise is Just Love Food Company. Their products are in most supermarkets but they don't do muffins. They do a traybake in Sainsbury's.
I'm down in the class party list for getting 3 x packets of Fabulous Bakin' Boys mini muffins... but they no longer exist! I know ty were the preferred supplier as they came from a guaranteed nut-free environment; are there any others who make the nut-free promise? Thank you!