I wonder about that, Rowan. In the U. S. and Canada, 1 part Potassium iodide is added to 10,000 parts salt (sodium chloride), eliminating goitre and thyroid problems and drastically reducing cretinism. I thought that this was the requirement for salt used commercially in food as well as sold as table salt. Anyone know the regulations?
I know some people claim that iodized salt has a metallic flavor. Sea salt has many elements, iodine among them.

In some countries, 1::100000 is the ratio, and some medical studies suggest that this is sufficient. Some foods (Maize or corn, Brassica greens, tapioca) increase the amount of iodide that should be ingested.

I can't find any requirements for the UK, but haven't really looked.