Re a two-year guarantee, well if that is correct, it's funny then that there are a lot of 1-year warranties for many electrical items... but my point is really is that the most common fault with an e-reader is some kind of screen damage, which wouldn't be covered in any warranty. I know Amazon have been reported to exchange within the one-year warranty they give on Kindle's for this kind of damage, I'm not sure if it's done as standard or not though, nor am I sure what Kobo, Sony or any other would do for something like a cracked screen?
Anyway, great to hear that WH Smith sorted you out with a replacement pretty quickly. It's not ideal that you had to wait a few days, but it's not like you could use the faulty e-reader in the meantime, and that's a pretty quick turnaround. Hope you have better luck with your new Glo.