When is a hallmark a legal requirement?

The current legislation is the Hallmarking Act 1973, which has been amended by Hallmarking (Exempted Articles) (Amendment) Order 1975, the Hallmarking (International Convention) Order 1976 (as amended), the Hallmarking (Exempted Articles) Orders 1982 and 1986, and the Hallmarking (Approved Hallmarks) Regulations 1986. Further changes to the legislation were made by the Hallmarking (Hallmarking Act Amendment) Regulations 1998, to bring the UK legislation in line with the rest of Europe.

The above laws apply to EVERYTHING sold in the UK, that is made of gold, platinum, silver and sterling silver, regardless of where it may have been manufactured and whether it does, or does not contain stones or gems.
The only exemptions, (covered by the above exemption laws stated above), are items that fall beneath the specific weight thresholds.

The thresholds are 1 gram for gold.

7.78 grams for silver.

0.5 grams for platinum.

If you sell an item of gold that weighs more than 1 gram, or an item of sterling silver that weighs more than 7.78 grams, or an item of platinum that weighs more than 0.5 grams, the piece by law MUST be hallmarked.