4wd - Thank you very much for the detailed explanation! All the advice in this thread was indeed very helpful. Thank you all!

I'll be getting an HP ZBook 14 G2 Mobile Workstation instead.
-dr_andus
That looks like quite a machine. If it is anywhere near as heavy as my Toshiba I recommend caution when slinging it over your shoulder.
-MilesAhead
It hasn't arrived yet, but it's a 14" machine weighing around 1.7 kg (the upper limit of the ultrabook category), which is pretty much the size and weight of my HP Chromebook 14, which I've been carrying around in my backpack. I don't have far to go, so for my use case it is manageable.
I did consider the lighter ultrabooks, especially the Dell XPS 13, the HP Spectre x360, and the Lenovo ThinkPad T450s, but at least in the UK they didn't come with big enough hard drives and/or RAM, and/or were a lot more expensive. I also wasn't sure if some of my favourite Windows apps would be usable on such super high resolution screens. Finally, I'm not ready to make the step up to Win8 and Win10.
What attracted me to the HP ZBook 14 is that it is still relatively portable (compared to the 15.6" and 17" laptops and workstations), while remaining powerful with a lot of storage, and that is easily expandable. Apparently there is a bay inside for a second hard drive; all drives and the battery can be easily swapped (and of course extra RAM added).
The weaknesses are the lower resolution (1920 x 1080 14" ISP screen), apparently the keyboard is mediocre, and it's not as much of a looker as the aforementioned competitors. But I can live with that, considering what I'm getting for the price.