An upgrade below that requires new Mobo and RAM if going to Ryzen or 8th gen.
Here:
PCPartPicker part list:
https://uk.pcpartpicker.com/list/LtGZTH
Price breakdown by merchant: https://uk.pcpartpicker.com/list/LtGZTH/by_merchant/
CPU: AMD - Ryzen 5 1600 3.2GHz 6-Core Processor (£137.99 @ Aria PC)
Motherboard: MSI - B350 PC MATE ATX AM4 Motherboard (£69.99 @ Ebuyer)
Memory: Team - Vulcan 8GB (2 x 4GB) DDR4-3000 Memory (£79.99 @ Overclockers.co.uk)
Total: £287.97
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-04-10 12:00 BST+0100
DDR4 2 x 4 GB at 3000 Mhz and Ryzen 5 1600. Will do well for now.
Or
Intel:
PCPartPicker part list:
https://uk.pcpartpicker.com/list/Mpnn7W
Price breakdown by merchant: https://uk.pcpartpicker.com/list/Mpnn7W/by_merchant/
CPU: Intel - Core i5-8400 2.8GHz 6-Core Processor (£146.99 @ Aria PC)
Motherboard: Gigabyte - B360M H Micro ATX LGA1151 Motherboard (£63.97 @ Scan.co.uk)
Memory: Team - Vulcan 8GB (2 x 4GB) DDR4-3000 Memory (£79.99 @ Overclockers.co.uk)
Total: £290.95
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-04-10 12:03 BST+0100
Go Intel if only gaming. But, essentially, both should give you near identical performance.