There are several factors in this development. Firstly, it can be considered as a pre-emptive move from Microsoft to head off the Chromebook challenge. Whilst Chromebooks are very limited in comparison to Windows laptops, they are not without their attractions and typically sell for the £200 mark. Sales of Chromebooks are encouraging but still modest, but this move may help prevent further growth. To achieve this, Microsoft have reduced the licensing cost to manufacturers for Windows 8.1. As a condition of this, Internet Explorer is pre-configured to use Microsoft's Bing as its home page and search engine, thus driving advertising traffic. Of course, this can easily be undone with a couple of minutes effort.
Secondly, many of these laptops use low-cost processors. These processors are often of the SoC ("System on a Chip") variety, in which the processor, graphics and other functionality are combined into a single chip - less components means less cost. Finally, the exchange rate of the pound against the dollar has improved, which helps reduce the cost of imports such as computers.
Not withstanding the comments in the opening paragraph, some people may be concerned that the processors in these laptops are just too slow. The Dell model cited has a respectable dual core Celeron clocked at 1.8 GHz, but other models may have AMD chips running at just 1GHz. The key thing to remember with consumer computers is that the main bottleneck on performance is not the components - it's the 'bloatware' (junk software) that comes pre-loaded on them. For instance, many come with trial versions of anti-virus software and these can run at 90% CPU utilisation for long periods of time. Removing it and switching back to the perfectly adequate built-in anti-virus program in Windows 8 that is absolutely free can transform the performance and perception of the computer.