I find this to be an almost humorous question. Somewhere in the neighborhood of 40-60% of all online gambling income comes from the US (depending on who’s numbers you believe).
The UK is probably a decent source of gamblers, but is tiny compared to the US. As for potential from other countries, think about places that have a large population with a good deal of expendable income. Australia and Ireland come to mind for the expendable income part. The Irish love to gamble BTW. China and Japan come to mind for the sheer number of people.