tsuk - I think the difference in between what you and Josie are talking about is the UK "Home Store" now is sometimes at
http://www.amazon.co.uk for some users. Currently UK users have a choice between registering their Kindles at the .com or the .uk site. When Amazon detects a UK IP address being used it makes a suggestion that they might want to migrate over to the .UK site but as of yet they don't force that migration.
So in effect it complicates the matter, some UK users consider their home store to be amazon.co.uk where they pay in British pounds while others consider their home store to be at amazon.com with a British address on the Manage My Kindle page where they pay in US Dollars.
Brazil doesn't have their own separate website for Kindle - so they always remain at the amazon.com web address. The choice you have in deciding book availability is determined only by that address on the Manage My Kindle page.