The reality is that, just as no man is an island, so is no country. Every government is constrained, formally or informally, by a whole raft of treaties and informal understandings. A good example is NATO which has enormous powers over our defence forces. To say nothing of the many US bases on UK territory, which are pretty much a law unto themselves, thanks to the Visiting Forces Act.
A government with absolute and complete power would mean living in the equivalent of North Korea - a pariah state that barely trades with anyone.
I don't want to be boring, but essentially Brexit was a con. Its promoters put something forward that was impossible in the real world. They did this (in my opinion) for nefarious reasons.
In the film A Man For All Seasons, Thomas More tells Thomas Cromwell that he has made the mistake of telling the king what he can do, not what he ought to do. The same mistake has been made, but in the modern world, 'the king' has been replaced by the voters. Or, under FPTP voting, the largest minority of them.