Used these to stock up on 4200 points the other day. Took a couple of hours to turn up, which seemed a long time for these things, but didn't find any cheaper after a quick half-arsed search.
I'm not sure what exactly are the cheapest points, but the ones I went for were here:
edit: The above are 4200 points for £29.99.
They are instant email delivery, which I prefer to having things delivered and, well, that I can be found out about basically!
Their 2100 points are £15.99, which I'm sure are cheaper elsewhere.