Dual Channel RAM is better than Single.
Go i5 8400, better than the 7400 and drop the CPU cooler because the 7400/8400 comes with one
PCPartPicker part list:
https://uk.pcpartpicker.com/list/jRsFTB
Price breakdown by merchant: https://uk.pcpartpicker.com/list/jRsFTB/by_merchant/
CPU: Intel - Core i5-8400 2.8GHz 6-Core Processor (£153.95 @ Aria PC)
Motherboard: Gigabyte - Z370P D3 ATX LGA1151 Motherboard (£89.98 @ Ebuyer)
Memory: Crucial - 8GB (2 x 4GB) DDR4-2400 Memory (£76.64 @ YoYoTech)
Storage: Western Digital - Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive (£35.99 @ Aria PC)
Video Card: Palit - GeForce GTX 1060 6GB 6GB Dual Video Card (£309.99 @ Aria PC)
Case: Cooler Master - MasterBox MB600L ATX Mid Tower Case (£61.28 @ Amazon UK)
Power Supply: Corsair - CXM 550W 80+ Bronze Certified Semi-Modular ATX Power Supply (£68.79 @ Alza)
Total: £796.62
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-03-02 11:56 GMT+0000
Due to insane GPU and RAM prices, that is the best you can do. No SSD, but has a 1060 6GB.
Also note:
1) GPU release next month so might want to wait.
2) You can get an unactivated windows to save money.
http://www.tomshardware.com/forum/id-3294531/downsides-running-activated-windows.html