I'm not certain of the manufacturers notafish but I understand that undercover investigations have indicated that some of the teething powders have been traced back to the UK.
Mercurial teething powders remain exceptionally cheap to manufacture and I have little doubt that they are sold with a huge mark-up at their final destination. Nevertheless, exposure and processing of mercury salts such as calomel remains an extremely hazardous occupation with strict operational guidelines from the HSE that have to be observed. As a result I remain of the opinion that these working conditions would hinder a major manufacturing company from marketing the product. It just wouldn't be financially viable.
I'm therefore inclined to think that the majority of the samples traceable to the UK remain a product of back-street laboratories.