While stocks last (which judging by the prices won’t be long) Amazon are currenty offering 4200 xbox live points for just £25.99 delivered. This price is a good £3-4 cheaper then other places making it a bargain deal.
Microsoft xbox live points can be used to purchase all sorts of items over xbox live such as:
Arcade games
Gamerpics
Additional game content
Movies
Xbox original games
More information on this great offer here