What is the practical difference for you between the sites? Do the books ship faster and cheaper for .co.uk? I wonder why wouldn't Amazon's backend just figure it automatically and just ship locally anyway.
* Some books will exist in one locale but not in others;
* Some books will be in stock in a locale but not in others;
* Some books will have a different cover and/or slightly different edition if it's a UK or USA version;
* If you are buying the Kindle version, availability varies by country, but only UK residents can buy from the UK Kindle store.
* If you buy many books from the States and you live in the UK, you'll end up paying hefty import charges.
* Delivery from your closest Amazon is usually faster (provided the books are in stock) and free (or cheaper for expedite shipment).
If you go to the Staff Picks of the Week section of my site (http://anynewbooks.com/staff-picks/), you'll notice how under each book I'm forced to place up to 5 links (US, Canada, UK, Kindle international, UK Kindle). It may be slightly confusing, but in my experience international readers appreciate the effort (the rest of the world is quite sick of US only web apps to be frank).
Saw just after posting that a good few other people already offered the same information.
Sorry, probably should have either left it there or deleted it entirely.
If you refer someone to the UK store then they eventually end up on the US store to buy then you won't get the sale (afaik).
Why they don't merge it all? No idea, but I'd guess media owners have different locale ownership structures so they only acquire distributions rights for an X-based company shipping to customers-in-X, and so on.
Having said that, I only have ordered books from amazon.com, perhaps you are referring to ordering something else beause amazon.com has shipped to Canada for atleast the past 9 years.
Granted, I haven't used amazon in about 3 years, they may have improved their global database system.