are the gold coins lawful money? if so, the face value
should determine. Otherwize they are worth what the market
says they are worth.
When the British empire declared that it's Gold Sovereign was no longer legal tender, the price of gold was such that a counterfeiter in Italy was able to profitably produce "British Sovereigns" containing more gold than actual Bristish Sovereigns.
And that counterfeiter got away with it ...
From Wikipedia, at http://en.wikipedia.org/wiki/Sovereign_(British_coin) , we have …
Sovereigns were produced in large quantities until World War I, at which time the UK came off the gold standard. From then until 1932, sovereigns were produced only at branch mints at Melbourne, Sydney, Perth, Bombay, Ottawa, and Pretoria (except for some in 1925 produced in London as part of Winston Churchill's ill-fated attempt to return the UK to the gold standard). The last regular issue was in 1932 (at Pretoria).
Production resumed in 1957, ostensibly to prevent the coin being counterfeited in Syria and Italy.[5] Subsequent publication of treasury papers appear to indicate that sovereigns were widely used in pursuance of British foreign policy in the Middle East, and it was felt that the coin could not be allowed to fall into disrepute, as many individuals were receiving payments in the form of sovereigns for services rendered to the British government.