Honestly, I would hold onto your 4790k and just upgrade to a 1080ti. The 4790k is still a very good CPU. Additionally, if you are going to game at 1440p or 4k, the CPU becomes even less important as the bottleneck moved from the CPU to the GPU. If you were to drop a 1080ti in your current rig, the fps would be within 10% of the rig on your parts list. Not sure if a 10% bump is work $1k.
But, if you want to make the upgrade, here is a few changes to the list. I would go with a little fast RAM as the pricing is very similar. I would also go with the Z370 board for the 8700k because you will need it to overclock. The H370 boards do not overclock. I would also get a little better CPU cooler as the 8700k is pretty hot. I went ahead and added a little better PSU, because you will be building a high end rig and you should not pair it with a budget PSU.
PCPartPicker part list:
https://uk.pcpartpicker.com/list/9XD89J
Price breakdown by merchant: https://uk.pcpartpicker.com/list/9XD89J/by_merchant/
CPU: Intel - Core i7-8700K 3.7GHz 6-Core Processor (£282.99 @ Aria PC)
CPU Cooler: CRYORIG - H5 Universal 65.0 CFM CPU Cooler (£44.99 @ Overclockers.co.uk)
Motherboard: ASRock - Z370 Pro4 ATX LGA1151 Motherboard (£92.95 @ Aria PC)
Memory: Team - T-Force Vulcan 16GB (2 x 8GB) DDR4-3200 Memory (£149.99 @ Overclockers.co.uk)
Storage: Samsung - 960 EVO 250GB M.2-2280 Solid State Drive (£89.87 @ Amazon UK)
Storage: Western Digital - BLACK SERIES 2TB 3.5" 7200RPM Internal Hard Drive (£101.99 @ Aria PC)
Video Card: MSI - GeForce GTX 1080 Ti 11GB GAMING X Video Card (£701.24 @ Amazon UK)
Case: RIOTORO - CR500 ATX Mid Tower Case (£44.93 @ Ebuyer)
Power Supply: SeaSonic - FOCUS Plus Gold 650W 80+ Gold Certified Fully-Modular ATX Power Supply (£89.99 @ Amazon UK)
Operating System: Microsoft - Windows 10 Home OEM 64-bit
Total: £1598.94
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-05-22 15:06 BST+0100