Silicilic acid, disodium salt - more or less is sodium metasilicate. Personally, I think this is one of the least concerning ingredients we would talk about. Yes, it provides alkalinity but it is miles safer than something like caustic soda/potash (sodium/potassium hydroxide) which you would find in the cheaper and more hazardous products (you are in the UK so have a look at some of the products popular over there, you would be shocked how many are based around caustics).
Butyl, IMO, is not a big concern either. Maybe if you were using it, routinely, at high strength, it would be an issue. However, most of the time it would be included at low percentage levels and then diluted further before use. Certainly in an APC, it would not be going to contact a surface at more than a fraction of a percent. I would be more concerned with other things (for instance the caustics). The optimum product noted has ingredients which are from the same family as butyl.
