After inspecting the free Microsoft points deal at my local tesco (monkspath), I found that they were included with the purchase of 800 Microsoft points (which tesco were selling for £10). I don't buy them personally; but as far as I'm aware, 1600 points for £10 isn't a bad shout.
I don't think it's possible to buy 1600 points in the UK, but looking around the next cheapest deals are;
1200 points for £8.49 @ 7dayshop (0.723p/point)
2100 points for £15.19 @ 7daysop (0.707p/point)
Microsoft are charging 0.850p/point.
1600 points for £10 works out at 0.625p/point, so is easily the cheapest of the 4.