This https://ukandeu.ac.uk/brexit-witness-archive/philip-hammond/ shows up how much the British government were winging their way through Brexit. Theresa May was mainly concerned about immigration rather than economics:

she will have seen this through the prism of immigration and security. For her, the economy would have been very much a secondary thing. She didn’t really have a deep interest in how the economy worked. Of course, she wanted a successful economy, because she understood that GDP growth underpinned everything else. But, as to the mechanics of it and what the implications were, that wouldn’t have been her primary focus at all at that time.

And when she announced her support for a hard Brexit, it was without consulting the Chancellor or having any idea of the economic implications:

I was completely stunned by the speech that she made at the Conservative Party Conference in October 2016. I hadn’t seen the relevant part of it in advance. I’d had no input to the speech. Nick Timothy kept me completely away from it. I did see some text on the economy the day before, but I had no idea that she was going to describe Brexit in th\e hardest possible terms.

I was absolutely horrified by what I was hearing. All I remember thinking was, ‘There will be a television camera that will be on your face. If you move a muscle, it will be the story on the front page of every newspaper tomorrow.’

[via https://davidallengreen.com/2021/02/how-theresa-may-casually-decided-that-brexit-meant-the-united-kingdom-would-leave-the-single-market-and-customs-union-the-fascinating-and-revealing-interview-with-philip-hammond/]