1. Your MoBo supports your CPU just fine (both have the same LGA1151 socket).
2. Can't help you here, not a Win8.1 expert. Sorry.
3. 144Hz monitor is good for FPS games. For other games, a 60Hz monitor will do just fine.
As far as your build goes, you've selected a K-series CPU but B-series MoBo. If you go with Z-series MoBo then you can overclock your CPU. But with B-series, you can't do that.
Tweaked your system little and here is the result.
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel Core i5-6600K 3.5GHz Quad-Core Processor (£213.34 @ Aria PC)
CPU Cooler: ARCTIC Freezer i32 CPU Cooler (£25.86 @ Amazon UK)
Motherboard: MSI Z170A SLI PLUS ATX LGA1151 Motherboard (£104.72 @ Amazon UK)
Memory: Kingston Savage 8GB (2 x 4GB) DDR4-3000 Memory (£53.99 @ Amazon UK)
Storage: Samsung 850 EVO-Series 250GB 2.5" Solid State Drive (£85.97 @ BT Shop)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive (£42.99 @ Amazon UK)
Video Card: MSI GeForce GTX 1070 8GB Video Card (£388.98 @ Amazon UK)
Case: NZXT S340 (Black/Red) ATX Mid Tower Case (£69.98 @ Novatech)
Power Supply: SeaSonic 660W 80+ Platinum Certified Fully-Modular ATX Power Supply (£136.99 @ Amazon UK)
Monitor: Asus VG248QE 24.0" 144Hz Monitor (£220.00 @ Amazon UK)
Total: £1342.82
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-10-10 01:47 BST+0100
Went slightly over the 1300 pound budget but here's what you get for the extra 42 quid.
CPU
Didn't change it.
CPU cooler
Put in Arctic Freezer i32. It has fluid dynamic bearing fan which is much quieter than the sleeve bearing fan used in 212 EVO. Fan itself will last about 3 times longer then the one in 212 EVO. CPU cooler comes with push system, but it can be easely upgraded to push-pull system with any 120mm fan.
specs:
https://www.arctic.ac/eu_en/freezer-i32.html
MoBo
As stated earlier, with Z-series MoBo, you can OC your CPU. MoBo also supports faster RAM than 2133 Mhz, up to 3600 Mhz and it has 4 RAM slots, including support for 2-way SLI and 3-way Crossfire.
specs:
https://www.msi.com/Motherboard/Z170A-SLI-PLUS.html
RAM
Switched it out for a faster sticks from Kingston, 3000 Mhz. Even though total RAM is 8GB, you have 2 RAM slots free for future upgrades.
specs:
http://www.kingston.com/dataSheets/HX430C15SB2K2_8.pdf
Storage
Didin't changed anything.
GPU
Changed the GPU to a MSI one. MSI GPU looks better in MSI board than Gigabyte's GPU.
specs:
https://www.msi.com/Graphics-card/GeForce-GTX-1070-ARMOR-8G-OC.html#hero-overview
Case
Left it as you picked it.
PSU
Switched it out for a Seasonic Platinum 660 (80+ Platinum, fully modular, Tier one)
specs: https://seasonic.com/product/platinum-660/
PSU Tier list:
http://www.tomshardware.co.uk/forum/id-2547993/psu-tier-list.html
Monitor
Didn't change it.