Dealer's Notice
UK law (the Hallmarking Act 1973) requires anyone selling precious metal items to display the official Dealer's Notice explaining what hallmarks mean. Here it is.

Download the Dealer's Notice (PDF). Notice produced by the British Hallmarking Council and distributed by the UK Assay Offices.
Items we describe as gold, silver, platinum or palladium are hallmarked or legally exempt (for example under the exemption weight, or made before 1950). Unhallmarked pieces that aren't exempt are described as yellow metal or white metal. Each product page shows its hallmark status.