In Aus there is a standard for all compounding chemists, it is illegal to sell products , even on the internet , that do not comply with the standards of practice. I would imagine that you would have a standards monitoring body in the UK, that would perform the same role. The compounding chemist I buy my Lugols from, is required to submit samples of compounds they produce for independant testing, to ensure they are providing what they say they are. I don't know what the labeling laws are like over there, but here they are very strict. It pays to have a good look at the reputation of any manufacturer before purchase.