I’ve also got a Nook, I got it a couple of years ago for about £30 and it’s similar to a Kindle but doesn’t support Kindle format books natively, although there are workarounds for this. It misses out on the simplicity of Amazon’s ecosystem too.
I believe Nook pulled out the market in the UK, transferring the content to Sainsburys, who also pulled out last month and transferred their content to Kobo.
I think the main players in the UK now are Kobo and Amazon, Amazon are much larger and their integration is seamless, but you are tied to them. I’ve not bought anything from Kobo.