Some more testing with the uk.nook.com/store website.
Using a free UK VPN (Securitykiss) I have created an UK account. Using a different e-mail address than my US account, but using the same Dutch credit card. Registering the account with an UK shipping and billing address.
I was able to buy free as well as paid books without a problem. One thing I noticed is that the Nook library has no option to download an ebook, just to archive or delete it.
I installed the Nook for PC program from the US website. The program works fine with the UK account.
I am no longer using my NC (bought an Android tablet) so I erased and deregistered it, than registered it to my UK account. I got an error about credit card info not detected, and the shop still points to the US store, but the books from the UK account were downloaded fine, and I could read them without a problem.
So non-supported devices (N1E, NC, NT) can be used to read the books from an UK account, but not to shop.