What are the best parts for gaming for £400 (UK)? This must include operating system?
http://uk.pcpartpicker.com/p/w8JYmG
Something like this? It's a pretty decent gaming PC, and you can make some easy additions to it. Should run a good number of games on medium-high settings, some on ultra at 1080p.
You might consider spending the extra £ on an R7 265, or an R9 270. You can get a decent R9 270 for about £30 more. The 270 will do a good job of running most games on ultra presets.