London Hallmarks. London has been the most important British Assay Office for the high quality of workmanship and the amount and variety of silverware passing through it. The Goldsmiths Company was the first office in England, authorised to assay and mark gold and silver. This followed the granting of a Royal Charter in 1327. The London Assay Office is still in operation today. The Assay Mark for London is a leopard’s head.