Saw this yesterday while on the xbox dashboard (on the Spotlight tab). There's an offer which says 50% off Xbox Live making it 7.50 for 3 months.
makes it about as cheap as buying 12 months at £30 from amazon.co.uk using the other deal ;)
Update: MAKE SURE you cancel after 3 months so that MS can't charge you the regular £15 afterwards :)
Ok just to clear things up, summary of most comments:
-you can't cancel online, you have to call and do it over the phone
-the deal is only for silver members, 9some gold members say it shows up on their dashboard so worth a try ;)
-stefone said he has quite a few posts on xbox live subscriptions so be sure to check them out for more info