As I understand it the reasons for potassium bromate being legal in the US are more to do with the idiosyncratic way the FDA treated things already in common use when it became responsible for regulating food carcinogens. So even a freewheeling Brexit means Brexit UK regulator is unlikely to approve this _today_; if potassium bromate were a novel ingredient today the FDA probably wouldn’t approve it either.
But the reason to be worried about UK standards is because the government may want food regulations to be as similar to US regulations as possible, to make it easier/cheaper for American food to be imported here. So the fact that the FDA wouldn't approve it if it were novel today doesn't necessarily mean the UK won't approve it purely because it's already approved in the US.
Aligning UK food standards with US ones would be extremely difficult, expensive, and unpopular, and would shut down most of the UK's remaining food trade with the EU and other places which align to EU standards. And potassium bromate would be the least of their worries...
That’s literally what successive UK governments have been saying for the past 6+ years.
There is literally a bill about to be passed by the government whose only purpose is to destroy all laws aligning the UK with the EU that would prevent trade with the EU for the purported purpose of being able to trade with non-EU countries.
