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Also worth saying that high Gini also creates high class borders, which means the ultrarich feel more loyal to their global clique than they do to their own homeland.
(This is what the "we'll move if taxed" actually boils down to. Also the classic "how do we stop the guards from killing us at the nuclear bunker" -dilemma.)
Low Gini leads more connections between classes, which leads to higher loyalty to your society, which leads to acceptance of higher taxes.
The above, btw, should pretty much be the ur-reason for the Nordic model.
Even with only barely profitable fishing industry, Norway had created a health care sector that drew a rich executive of a major company to abandon his career, and move to a place with less than welcoming climate. This led to Norway having the know-how to avoid the Resource Curse, pretty much the only country to do so.
In other words: high Gini may create nice palaces, but it doesn't ensure healthy people.
The North Sea oil fields are shared between several states, so it isn't the difference between UK and Norway.
Bigger part, imho, is how a former officer of the British Iraq Petroleum Company named Farouk al-Kasim moved to Norway due to his son needing treatment for cerebral palsy.
He applied for a position at the ministry of industry at the same time as the Fields were found, and ended up drafting almost singlehandedly the Sovereign Fund, which he hoped to solve the Resource Curse.
Comission (that is, the EU-ministers) are. The other branches are not.