I bought a NookColour and a US$25 gift card. I can download & read free & sample ebooks, but I can't purchase eBooks using the gift card.
Copyright restrictions prevent B&N from selling eBooks outside US territory and, to a very limited extent, Canada.
If you're in the UK, try Waterstones or WH Smith -- they both have extensive offerings. In Canada, Kobo's for you.
Good luck, and happy reading!