Indeed, as unless you really want the keyboard to make notes, you can get pretty much everything in the
Cybook Gen3 as the kindles wireless is not going to work in the UK.
The other reason, the Amazon E-book store (which is only really an option if you have a US credit card) while having great prices is more or less comparabe to the prices you can get at sites like booksonboard and Fictionwise, even if you may have to comparison shop at times to get the best deals.
As for availability of books, while Amazon probably does get them first, the others shall probably get them soon as well, as Mobipocket is an amazon subsidiary.
The advantage of getting the Gen 3 is that you can easily buy books from a number of stores without having to worry about whether you have a US credit carf, or getting gift cards. Also, you'll get the full warranty as Bookeen is a European company.
Finally the Gen 3 has support for TTF fonts, so you can load the font you are most comfortable with for reading onto it. Also it supports multiple dictionaries at the same time, something the kindle does not currently do.It is also the lightest of the e-ink readers available.
Pricewise, while the Gen 3 is usually the most expensive,
WH Smith is advertising one for £180 which includes a case, making it the best deal in the UK. And it appears to be in stock right now so if you want it you can order it right now.