Do you need to use Calibre to transfer Amazon UK ebooks to an existing Kindle account (Amazon US)?
Where else are people buying cheap (legit) ebooks?
I don't think you can transfer books between Amazon accounts, but I'm not sure.
With Calibre, all your ebooks regardless of source are treated as one library. You then choose which books to put on which device(s).
One feature of Calibre is a search engine, so you can specify a book by title/author and it will show all sources to buy that book from.