The UK's government is driving the UK off a cliff. What do they think they are doing?
In the case of Trump this is easy to answer: he wants to be dictator, for his family to rule after him, and given his vast corruption he is completely terrified about what will happen to him if he loses power and the protections that come with it.
For the UK the conspiracy theorists' answer is that Cummings, Johnson and the tories wish to simply extract all of the money from the UK as the tories ever have done. I think this isn't true: if (when) the UK falls they fall with it. But they can't believe that the no-deal brexit that is now all but a certainty will be good for the UK, or for them, can they? Are they that stupid? Perhaps they are that stupid.