this needs to be clicked through the link on 3g not wifi for free spotify premium when using the 1 plan on 3 im not sure if this applies for sim only AYCE data plans as well
Credit to sunnygill for the following clarification:
Spotify and Three have come together in an effort to bring this revolutionary way of finding and enjoying music to your mobile phone. All 3 One Plan customers can try Spotify Premium free for two months by simply texting ‘Spotify’ to 39000. That's two months of unlimited free music wherever you are! This great deal is available for all customers with an All You Can Eat data plan.
Got the above info from:http://www.store-3.co.uk/free-spotify-premium-trials-on-three.html