Best I could do while keeping it an actual decent PC;
PCPartPicker part list:
http://uk.pcpartpicker.com/list/yf4Cr7
CPU: AMD Athlon X4 860K 3.7GHz Quad-Core Processor (£67.55 @ BT Shop)
Motherboard: Asus A88XM-A/USB 3.1 Micro ATX FM2+ Motherboard (£63.26 @ BT Shop)
Memory: Corsair 8GB (1 x 8GB) DDR3-1600 Memory (£35.99 @ Amazon UK)
Storage: A-Data Premier SP550 120GB 2.5" Solid State Drive (£35.63 @ Amazon UK)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive (£41.99 @ Novatech)
Video Card: PowerColor Radeon RX 460 2GB Red Dragon Video Card (£98.99 @ Ebuyer)
Case: Fractal Design Core 1100 MicroATX Mini Tower Case (£29.99 @ Amazon UK)
Power Supply: Corsair CX 430W 80+ Bronze Certified Semi-Modular ATX Power Supply (£49.74 @ Aria PC)
Operating System: Microsoft Windows 10 Home OEM 64-bit (£84.98 @ Novatech)
Total: £508.12
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-10-23 12:30 BST+0100
Windows 10 can be swapped out for 8.1 to save £55-ish. Budget is a bit tight to want to play witcher 3 on anything but low-medium. Also assuming you didn't need a monitor, keyboard or mouse.