It’s not clear if Theresa May’s blustering on hard brexit is anymore than just that. It might be a negotiating tactic to try to retain membership of the single market.
Even so, it’s still pretty stupid.
Even ignoring the fact that every time she opens her mouth the pound loses value, the very fact she seems happy to head towards a hard brexit will be spurring a lot of international businesses in the UK to be drawing up very concrete plans to relocate a lot of jobs to other parts of Europe. They might even go through with these before a hard brexit is unavoidable simply because part of the draw of the UK is a reputation for stability, which Theresa May has damaged even without any real actions.
Or in other words, Mrs May is doing her best to make us incur negative effects of brexit regardless of whether we actually need to.