There's no need to change your location from UK to Spain. Indeed, if you do so, some items will no longer be available. For example, if you subscribe to newspapers, magazines or whatever then your order will be closed and you will have to take out a new one with Amazon.es which is more expensive. Same with if you order books, the ones available from .co.uk will now be the ones from .es even though you can scroll through but the prices will be different.
Simply leave your location as UK. If you use Google Chrome it will ask if you want to translate to English. Just tick the box "always translate" and it will do it for you. Alternatively, just press the right button on your mouse and select translate to English.
You can also sign up for Prime Spain even though your location says UK.