Jimmy Sierra comes over to the UK every year and has done for a few decades,he brings a party of detectorists over for a 2 week detecting trip but this is just normal detecting and not specifically gold detecting and that is a massive differance,yes some gold items have been found most of these would be say celtic gold coins that type of gold item.It has nothing to do with say nugget hunting or anything like that,most of the land that he has detectings digs on in based around say the Norfolk and Colcester area as this was prime Roman and Saxon land.
I guess most of this land that he has every year has been un-detected before,but he is in a position due to the prices he charges for his detecting holidays to pay landowners/farmers premium prices to detect on the land,basically if someone offers you alot of money to detect your land i doubt that you would turn it down.
One thing i will say that although the history on the land that his visit go on can go back 1000s of years he seem to find gold items on most visits every year,in the real world this does not happen i can assure you,it must be pure luck or some divine intervention