Hi Arianne and welcome to MR from a fellow Brit.
In terms of which one to get, have you seen this
page on the Wiki? It will at least show you which can handle PDFs, which can allow you to annotate etc at a glance.
Following that, my suggestion would be to physically try out as many of your short list as you can. The preference for look and feel of these devices does seem to be a very personal thing, so nothing beats a hands-on look.