Yes, yes I know we're already kind of there, but I mean without the current infighting and clash between finance vampires and industrial vampires and other gridlock factors. 
Fascism burns itself out fast with the constant need to purge and loot until nothing is left, so I know even 20 years of unmitigated fascism is a stretch. Still, what do you think a "they get absolutely everything they want, fire all US government employees and replace them at the low echelon with Billy Bob, the middle echelon with James Holden Bloodfeast III, and their favorite Youtubers at the top echelon" complete and total MAGA victory might look like? 
Try to hold it together and stretch your sense of disbelief that the hogs get the feed they've been squealing for, from mere reappropriated resources (the way nazis murdered and looted and redistributed to their favorite failfamilies) or even something surreal like the incel supporters actually getting government issued slaves while destroying all other safety nets entirely, if you dare. After all, Dr. Professor Lobster Esquire PhD MD says that's the price of civilization, bucko. 
Let's be generous and assume no outright nuclear war, just so there's something to actually visualize other than a more actually historically accurate version of the Fallouterino treats. 
Not much change even with a clumsy Brexit-tier unraveling of the admittedly-shitty status quo at a bureaucratic lvel?
I think the things that they care about would continue while the things they don't would get taken apart, but in a more rapid fashion than the "starve the beast" approach that there been doing for half a century.
So, not much change?
What would we even brexit from? NATO? It's us and our goons armed with our guns. The UN? The security council suddenly becomes somewhat more effective.
What I meant by that is a big clumsy mass upheaval without any thought-out plan, like Brexit was.
Purging all government offices would have so much brain drain, sabotage from rightfully pissed workers, and other factors I can't even fully fathom, and having a bunch of sycophantic chuds bumblefucking in all at the same time would be a circus fire of carnipocalyptic proportions.
Purging all government offices would be even more immediately apocalyptic than brexit. Like I'm pretty sure the UKKK still has some form of food safety regulation (for now)
I've heard from techbro bazingas, live and in person, that "government can't do food safety right" and therefore they'll just do it themselves.