In most European countries you are allowed to return defective products without the original packaging and used. You just can't return items that way only because you didn't like them.
In Spain, you can return things that you didn't buy in person (eg online, on a catalogue or via phone) within 14 days of receiving it for any reason whatsoever, as long as it's still in good condition.
I believe this has been EU wide rule for a while.
That's EU+UK wide, though there are reasonable exceptions like food & personalised (e.g. engraved or embroidered) goods.