Buy 2x 2100 Points for £17.95. The single 4200 unit costs £36.79 so don't go for that.
Shipping is an additional £0.99
Use the voucher code 'WEEKEND15' for a 15% discount [thanks go to original poster, 'DisneyFan' for this code]
In additional to this use TopCashBack or Quidco for another 10% saving [£3.15 excluding shipping] which makes the 4200 Points £28.36 total!
4200 Points will typically cost you around £32-33 from retailers such as Play.com and HMV for example and additional cashback percentages are not as generous.
I've been looking for deals on Microsoft XBOX360 points for some time and this is the cheapest I've found online for genuine UK points.