Ok, if you really really want to buy from this company, here is what I would do:
Go to that custom build here:
http://www.chillblast.com/pconf.php?productid=18376
Leave everything at default except if you want to spend 20 pounds for a bigger harddrive and maybe change the case to your preference.
That puts you at 383
Then go and buy your videocard separately and snap it in. You can get a kick ass videocard for that difference.
The best bang for the buck videocards right now in that range are the Radeon 7870 and the Radeon 7950. Look at prices on Amazon for them. Buy a 7950 if you can afford the cheaper ones. If not, a 7870 is still really good. Both are good cards. They also both come with 3 free games (Bioshock Infinite, Tomb Raider and Blood Dragon for 7870 and Bioshock Infinite, Crysis 3, and Blood Dragon for the 7950).
Still, you would be able to build a better system for significantly cheaper if you picked out individual parts. There a lot of people here and a lot resources that can take you through step by step and it takes an hour or two to put together. Mostly like snapping lego blocks in place.
Those 3 Chillblast you linked to above are DEFINITLELY far better than that HP computer. Do not buy that whatever you do.
Basically for your budget, I would say make sure you have a processor that is an intell i5 (the i3s are only dual cores) and look for a Radeon 7870 or 7950 graphics card to put in it. Graphics cards are easy to install and those places are charging you a premium for old cards. Better to just avoid adding a videocard to their builds and just buy one separately to put in yourself. Again, that's if you are really set on not building from scratch.