Ryzen 7 1700:
PCPartPicker part list:
https://uk.pcpartpicker.com/list/QGnYhq
Price breakdown by merchant: https://uk.pcpartpicker.com/list/QGnYhq/by_merchant/
CPU: AMD - Ryzen 7 1700 3.0GHz 8-Core Processor (£236.95 @ Aria PC)
Motherboard: MSI - B350 TOMAHAWK ATX AM4 Motherboard (£84.98 @ CCL Computers)
Memory: Team - Dark 16GB (2 x 8GB) DDR4-3000 Memory (£188.99 @ Overclockers.co.uk)
Total: £510.92
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-02-16 18:08 GMT+0000
Ryzen 7 1700x:
PCPartPicker part list:
https://uk.pcpartpicker.com/list/FBjYzM
Price breakdown by merchant: https://uk.pcpartpicker.com/list/FBjYzM/by_merchant/
CPU: AMD - Ryzen 7 1700X 3.4GHz 8-Core Processor (£246.00 @ Aria PC)
CPU Cooler: Corsair - H60 54.0 CFM Liquid CPU Cooler (£63.95 @ Aria PC)
Motherboard: MSI - B350 TOMAHAWK ATX AM4 Motherboard (£84.98 @ CCL Computers)
Memory: Team - Dark 16GB (2 x 8GB) DDR4-3000 Memory (£188.99 @ Overclockers.co.uk)
Total: £583.92
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-02-16 18:09 GMT+0000
Both are 8 cores, so two more cores than the 8700k, but with slightly worse single core performance. If you are doing video editing, intensive CPU work I would go Ryzen. It is also cheaper. That said, 8700k is still a force to reckon with, and the way to go for Gaming. It can also stream and game pretty well.
The Ryzen build is cheaper because I gave it a high end B350 mobo, if you go X370 it would be same price or a bit more.