For what it's worth, I live in Central America but have a US mailing address and credit card. I bought my Nook HD+ a couple of months ago and had it shipped to the Miami address of a mail forwarding service I use.
I had no problem setting up a US-based account with B&N and registering my HD+ to that account. I've since purchased several books and magazines from them without any geo-restriction problems. I haven't tried to buy any apps from B&N, since I have full access to all the Google Play apps so I'm not sure how the Nook app store works outside the US. I'm not sure about opening a UK account but you should have no problem opening a US account and registering your Nook to it so long as you have a US address (even a temporary one) and credit card. You can use a prepaid VISA card for this.
If you decide to root it, I don't see any compelling reason to register it at all with B&N. There's nothing they sell in terms of content that you can't get elsewhere. I buy most of my ebooks from Amazon and read them in the Kindle For Android app on my HD+.
Although many people root their Nooks themselves, I think it's easier for most non-techies to buy a preconfigured sd card from AFN or N2A. They're available in dual-boot configurations or as replacements for the stock Nook OS. I like the dual boot card from AFN. It works perfectly and allows me to switch back and forth from Nook OS to Android without voiding the warranty or making any changes to the stock OS. If you want to use the SD card for storage as well as booting into Android, get at least the 16GB card or, better yet, the 32GB. You can buy the AFN cards from Amazon or directly from the developer:
http://www.androidfornook.com/nook-shop