There are some theories in the UK that are close to conspiracy theory.
Many leading brexit proponents are rich people who made their money from the collapse of the soviet union and have assets largely outside the UK. One conspiracy theory is that they are attempting to crash the UK economy in order to buy the dip, make money from shorts and the sell off of state assets.
Another conspiracy theory is that brexit has an ideological basis in the idea of the soverign individual - related to the idea of the randian hero, the idea is that as states weaken in power, rich people will be able to transcend citizenship and attain some kind of libertarian utopia where they can do what ever they want. One of the leading brexiteers dad wrote a book about it which apparently has some following, but maybe people are just surprised that right wing politicians want a smaller state.
There is also an idea that the UK and its dependencies (e.g. virgin islands) are a tax haven. A lot of brexit money came from US activist billionaires and hedge funds, but a lot of it came from very rich russians who fled tot he UK in the 1990s with huge amounts of money that Russia say was stolen visa corruption and other criminality. The theory is that brexit was a response to EU money laundering and tax evasion rules that would have linked these rich criminals to their money to its sources, leaving them open to russian legal challenge. The theory is that for those people, brexit was a means to escape the net.
In my opinion, the role these factors played is probably overstated - brexit was primarily a backlash against globalisation and change, and a protest against the UK status quo by parts of the population that didn't believe they benefited enough from it, much like the election of Trump in the US.