America can undo this with some effort, the UK won't ever get back into the EU. Or at least not within a potential 4 year turnaround, 8 at worst. So no, UK is still dumber than America.
America won't undo this for at least another 15+ years if not more (and that's assuming Trump is a one term president). The Supreme Court nominees could have an effect for over 30 years. The global warming effect might be permanent.
This is leagues worse than Brexit.