Amazon UK currently has a flash deal on for three month Xbox Live subscription codes for the low price of just £8.99. Considering the monthly cost is £5.99 and usually three months is £14.99, you stand to save yourself a good chunk of change.
Consider also that buying four of these will cost less than a single one year subscription right now, and you're definitely on to a winner. The deal is only valid until May 12, so grab 'em while you can!