Regardless of whether or not you can register your Romanian credit card as the payment method for 1-Click, there might also be geographic restrictions that might prevent you buying particular books.
These are restrictions that are imposed by the publisher, not by Amazon. The point is that if a particular title is unavailable in a given country, you can't buy it in that country, regardless of how you buy it or who you buy it from.
If your Kindle is registered in the UK, and you use your friend's account to buy books, then you will be able to buy any book that is available in the UK, which includes all the books on Amazon's UK Kindle site. But I don't know how many books would be available to you if you were able to use your own card to make the purchases.