At £4 for 3 months this would work out at £16 for a year on a pro rata basis making it the cheapest I've seen.
It is to be found at http://www.littlewoods.com
Just do a search on Xbox Live and add the 3 months item to your basket, in checkout make a new account and then use the following
promotion code, it will deduct £15 from your total bringing it to the £4
The Promotional Code is:
I've just done it and it worked a treat. Not sure how long the code will work for.