Most of the so called Ex-UK laptops are refurbs. For some manufacturers, e.g. HP, you can tell if a machine is a refurb by checking the serial no/part no. See
http://h30434.www3.hp.com/t5/Other-Notebook-PC-Questions/How-to-tell-if-notebook-was-refurbished-from-HP/td-p/932783
I have bought a good number of HP refurbs and they work extremely well. My checklist is usually simple:
a) If the OS is not genuine (Unataka Windows 8? Tutakuwekea. Hata ukitaka Windows 7 tutaweka), I steer clear. Most new laptops come bundled with genuine Windows licenses. Even if I am going to format and install Linux the next day, it's still a mark of legitimacy. If the machine has been broken in, I don't touch.
b) Register with HP. The next thing I do is register the laptop with HP. The laptop must be recognised by HP from the serial no.
c) As said above, if the rates are way lower than peers/competitors, think twice