GTX 1070 build fo £700

PCPartPicker part list / Price breakdown by merchant

CPU: Intel Core i5-6500 3.2GHz Quad-Core Processor (£174.99 @ Amazon UK)
Motherboard: Asus H110M-K Micro ATX LGA1151 Motherboard (£47.99 @ Amazon UK)
Video Card: MSI GeForce GTX 1070 8GB Video Card (£388.98 @ Amazon UK)
Case: Cooler Master N200 MicroATX Mid Tower Case (£43.45 @ Amazon UK)
Power Supply: Antec 550W 80+ Certified ATX Power Supply (£49.99 @ Amazon UK)
Total: £705.40
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-10-01 22:03 BST+0100

You'd be better off dropping to a 1060 though imo, & upping the quality of the board & getting an ssd in there.
 
Not really no, most current titles only use 2 or 3 threads & the 6100's hyper threading works well in the majority of cases.

Just when people have the budget for a 1070 they would normally stump the extra for an i5.

Its just not a combo you'll see used that often in all honesty as it goes.

What's the absolute reason for the 1070 ??
You do realise its complete overkill unless you're running a 1440p+ screen.