ok... from someone faced with a similar problem about a year ago... if you have a regular account with amazon.co.uk with a UK bank account listed as the default billing address then any Kindle that you purchase from anywhere will enable you to download books from amazon.co.uk and / or amazon.com.
If however your amazon billing account defaults to a Spanish bank account you can only download from the .com and .es sources. Books downloaded from the.com site are invariably more expensive than the same item purchased from the alternative .co.uk or .es addresses.
There is another alternative... unless you actually need to be able to read books whilst 'on the move' via a Kindle, amazon have a free application for PC and Mac computers. You can download this app from amazon.es.
As for the availablity of books purchased from the .es site, there are thousands of them... available in Spanish Italian, English, French and German languages. Most recently published titles are available in all language variants.
Go to amazon.es and from the menu select Kindle- Ebooks en idiomas extranjeras. On the new page that opens select 'Ingles'. Browse by titles displayed or enter the title or author in the Search window and then treat the whole opèration as a normal or 1 click purchase...
I use the .es site for Kindle books, I use the PC app and have had no trouble with downloads despite being in a fairly isolated area.
Hope this helps.
J.