It is my understanding that:
1) If you buy a Nook in the US, it will link to the US Nook Shop and requires a US credit card on the Nook account.
2) If you buy a Nook in the UK, it will link to the UK Nook Shop and requires a UK/Euro credit card on the Nook account. (don't buy a US model and ship it there)
3) If you buy a Nook from the BN website it will only accept US payment, and ships a Nook only to US addresses
You could work around this possibly by seeing if any of the retail partners at the UK website will allow you to pay for a Nook with a US credit card and ship directly to a UK address.
Hope this helps.
edit: you mentioned Nook ST, the good news for HD/HD+ users is that it asks you what country you are in at device registration, so less of an issue with those devices