The Kindle's main rival in the UK is the range from Kobo:
http://www.whsmith.co.uk/KoboeReaders.aspx
The Nook is produced by the American bookseller Barnes & Noble. It's been on sale in the USA for quite some time but it's a fairly recent arrival here in the UK:
http://www.currys.co.uk/gbuk/nook-simple-touch-ereader-black-18877434-pdt.html
and
http://www.johnlewis.com/nook-simple-touch-glowlight-ereader/p231759497
Sony has the Reader. (I wonder how long it took them to come up with that name!):
http://www.sony.co.uk/product/rd-reader-ebook/tab/overview
There's also the PocketBook range:
http://tinyurl.com/botdhds