I thought the psu was on the high side, but the last time I did any thing with my PC was around 4 years ago and I'm seriously outta the loop.
850W tends to be appropriate for a build with a top-tier graphics card, like a 7970 or 680. For a budget build like this one, it's hardly necessary, and as others have noted, it must be from a reputable brand (SeaSonic, Corsair, OCZ, Thermaltake to a lesser extent, and a few others) or else you risk some severe degradation over time.
The sum of the parts comes to about £450 so it would seem the value is there.
Also, the sum of those parts most definitely does NOT add up to £450. It's more around £350...