CheshireCat wrote:
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.

That seems to be exactly right, if I am reading correctly.
And it's the basis of my grumble. Suppose Mr. X makes a
really beautiful mokume ring with lots of platinum and
silver. Some of the swirls are pure gold.
Unfortunately, the ring contains some bronze and copper
streaks for contrasting color. So, let's say he can't
get it hallmarked.

Unfortunately for Mr. X, it is "an offence for any person
to apply to an unhallmarked article a description
indicating that it is wholly or partly made of gold,
silver or platinum."

This seems to me to be an injustice to Mr. X.
He *should* be able to attach a truthful description
to the ring.

It would be bad enough to slap Mr. X's wrist if he
refuses to comply, but he can also can get jailed for
*TEN*
years.

That makes the injustice much worse.
Mr. X has commited no fraud.
He has simply engaged in a bit of healthy civil
disobedience.