At some point, I came to my senses and bought a used Thinkpad X230 for just £80. I spent £100 on upgrading the RAM and SSD to match those of the XPS I mentioned (I got a rather lucky deal on the SSD). I'm currently planning on spending approximately another £100 on upgrading the display to a 13.3" 1080p IPS panel (which will require some fairly extensive hardware mods).
So, for about a quarter of what I was going to spend on the XPS, I have a machine with roughly comparable specs (Significantly weaker CPU, admittedly), but a much more rugged design with upgradable and moddable hardware.