First off, do NOT buy that power supply, it's not a good unit and is not a good idea at any price.
I'm really struggling to put together even a basic gaming build for £500, especially one with any degree of expandability for a graphics card upgrade later on to make it more capable. You'll need a case with decent airflow, a PSU that can handle it, and a decent board for starters, and that already blows a good chunk of your budget. Particularly, IMO, it's pointless buying a system these days without an SSD, because it just won't feel quick day-to-day even if it does perform well in games. A budget of £600 starts to put you into the realms of possibility.
I would agree that perhaps going used is the way to go, because for example it's possible to pick up a bundle of CPU (roughly on par with the CPU you have in your list), RAM & motherboard for £130 with ease. I'm somewhat skewed as I tend to buy very few components brand new due to the good deals possible when going for used parts, but equally I understand it's not a route everyone wants to go down.