Nothing. Harry is not likely to become King of the UK since he is currently fifth in line of succession and likely to go down the line as William, or eventually ,George and Charlotte, have children. Britain remains Britain and goes on as it does now.
Nothing since his older brother William, and his nephew and niece are in front of him in succession to the throne in the UK. Only by some fluke, that if something happened to them, he would ascend to the throne.
Yes, in theory he might marry a queen regnant somewhere in the world;but he would not be king of her country, he would be a consort. aS He is well down now in the line of succession, it would have no affect on Britain
Nothing,the british monarchy has all the succession sorted, charles, william, george, george's children etc,if harry left britain it would make no difference at all