But, the vast majority of the public don't buy the exclusive jewellery items
you talk about.
If you buy something of mixed metals like mokume then you'd be unlikely to
melt it down with a quantity of scrap gold. A hallmark at least shows the
whole item shows a certain purity and they're usually placed on non visible
surfaces.
Expensive, exclusive items are likely to pass onwards into history. They're
often accompanied by other paperwork to prove their ancestry.
Maybe there's different rules for jewellery made from multiple metals,
perhaps hallmarks aren't allowed for these pieces as they're not pure
enough.

extract from http://www.thegoldsmiths.co.uk/assay...ghallmaked.htm

ARTICLES OF TWO OR MORE PRECIOUS METALS
The hallmark applied is that of the least precious metal, which must be more
than 50% of the weight of the article. Whenever possible, the fineness mark
of the more precious metal is added.

a.. Platinum is more precious than gold, and silver is the least precious.
Platinum articles with less than 50% by weight of yellow 750 gold are
hallmarked as platinum.
b.. 750 white gold snaps, ear-ring fastening pins are permitted on
platinum articles.
c.. Platinum and 750 white gold articles are hallmarked as 750 gold.
ARTICLES MADE OF PRECIOUS METALS AND OTHER MATERIALS
Small, working base metal parts are permitted. Regulations on the use of
base metals are made from time to time by the British Hallmarking Council.

Authorised parts in base metal must either be clearly distinguishable in
colour from the precious metal or struck with the word "METAL" .

Base metals such as stainless steel are not considered to be distinguishable
from silver, white gold or platinum.