If you are referring to Brexit, I disagree. When it comes down to it Brexit happened because of a lack of consultation on the surrender of parliamentary power to Europe (treaties signed without Referenda first). This caused disenfranchisement.

A lot of the resentment towards Eastern Europeans has to do with the fact that "we" didn't invite them in, "Europe" foisted them upon us.
But why just Brexit? Populism has had trouble penetrating your national government over its history more because of inertia than any palpable pressure from the monarchy, because conservatives deferential to the monarchy have typically ruled over the national government, because your most significant populist movements have been ethno-national ones seeking dissociation rather than representation, because social authority was contested and compromised on the basis of landed aristocracy embracing market capitalism under state stewardship into the Industrial Revolution. These quirks, and their attendant norms, are smoothing out as time passes.

I don't think the monarchy continues to act as some primitive force distinguishing American results from British ones. Let's say the only discrete forms of democracy are party democracy, and populist democracy. Would you be surprised if there were a referendum towards dissolving aspects of the monarchy in your lifetime?