When we lived in England in the 1960s, my mother had a book on hallmarks and how to read them. It was fascinating. It never occurred to me that imported items would be reviewed and hallmarked appropriately.
Yes. All precious metal items over specified weights (different for different metals) have to be assayed and marked before sale. It's another cost (as it isn't cheap) which helps to make us uncompetitive for exporting, unless the buyer appreciates knowing for certain what they are getting.
It's easy enough to look up hallmarks for the date assayed and where and what. The who might still need a bit of research.
By law sellers must also display a hallmarking notice on their website.
I still feel awed when I look around Goldsmith's hall at the names of guild members going back for centuries. London assay office and Goldsmith's Hall is just by St Paul's Cathedral.
The assay office has super 21st century assay kit but most assay is still done by stroking the piece on a touchstone and comparing the smear with a known standard streak.
I just got some new makes back from the London Assay Office , and managed a half decent photo of the mark, including, at the bottom, the special late Queen's platinum jubilee mark which ends at the end of this month
Silver hallmarks have four main elements: the town mark ILondon - leopard's head) , the date letter (x), the maker's mark (that's me, WMG in that specific shape), and the lion passant (which is the mark for sterling silver)
A weekend of polishing and pearl setting awaits me.