Brexit/political uncertainty seems to be the excuse for everything at the moment. I would like to see what these sales are relating to, either business or personal sales. I would expect the business sales are probably staying reasonably constant, while the personal sales are declining as people buy phones/tablets instead. (Of course, there may be some companies moving overseas, but it's not like companies are leaving in droves).
Another thing which may contribute is SSDs. Where I used to work, the company used Lenovo equipment. Over the last couple of years, SSDs have become standard in the laptops we had from them, and we upgraded the older slow machines with SSDs in place of their conventional hard drives, and they became as fast of the new machines. The SSD machines don't seem to suffer from slow performance over time, so there was no need to replace them as frequently.
As the SSDs make a substantial difference to the device performance, companies may simply be choosing to keep their laptops (and some desktops) longer before they need replacing.