O2's Amazon music unlimited offer cannot be activated. Amazon does not recognize it. When trying go redeem the extra, I was transferred to Amazon's site and got an "Activation error" message. I tried to get support from Amazon and they couldn't help arguing they don't have the details of the offer and suggested that I reached out to O2. When I finally could talk to an O2 assistant, well, you guessed it, he told me that everything was ok in O2 and that I should reach out to Amazon. And closed the chat session on my face.
Well, it seems O2 is not willing to honour its offer. Shame on them.