Interestingly, as all populists are by necessity incompetent in the matter of governance (their only competence being the telling of infantile lies to their simple-minded supporters) the results are somewhat predictable. We see populist incompetence in the USA as the GoP devotes 100% of its effort toward destroying the nation. We see populist incompetence in the UK, this time amusingly self-destroying thanks to grotesque infantilism combined with extreme stupidity that is on par with anything Trump was capable of babbling. As things break and collapse, the supporters of populism will continue to buy in because they lack the ability to do anything else; others, however, want off the sinking ship.
The USA will, like the UK, come apart at the seams. There is no reason for the coastal States to go down with the bible belt Titanic. Although the SNP in Scotland is today too incompetent and feeble-minded to pursue devolution successfully, the next generation of SNP leadership will likely be less obtuse and all too ready to take advantage of English self-enfeeblement. And so both the USA and the UK will come apart and new smaller nations will emerge - and contend with each other bitterly.
The long collapse of the Roman Empire shows us how this inevitable pattern plays out; today, however, it's all going to happen far more rapidly thanks to our wondrous modern technologies.