A hallmark:

Is a set of component marks applied to articles of the precious metals gold, silver, platinum or palladium.
Means that the article has been independently tested.
Guarantees that it conforms to all legal standards of purity (fineness).
Guarantees provenance by telling us where the piece was hallmarked, what the article is made from, and who sent the article for hallmarking.
It’s a legal requirement for silver over 7.78g, gold and palladium over 1g, and platinum over 0.5g to have a UK recognised hallmark. Items below these weights are not required to be hallmarked.

I have my own UK Hallmark, which is applied by The London Assay Office.

To find out more about hallmarking, visit the London Assay Office here.