I can still use the US store. I've been swapping between the two to have a look. My location is set as US, and when in the UK store I just click on the "Not from the UK" link and go back to the US store. I think they're using cookies to identify you, but if you click to go back to the US store it overrides the UK setting.
I would say that the problem is that fewer ebooks are officially available in the UK - as for paper books there are different agreements for different geographical locations. What you see in the UK store is just what's available in the UK. When we buy from the US store, we're sometimes buying ebooks that are not available in the UK.
I had a funny thing with pricing. Because I had things on my wishlist from the US stores, when I moved to the UK site the prices were just converted to pounds, only when I added these books to my shopping cart did they show the UK price. One book went from £8.37 to over £18, which is definitely not all VAT.