What specs should I have?

[fastflight]

I want to play fallout 4 on ultra at 1080p/1440p and have an average FPS of 50-60. My budget is around £1000-1200 thanks.

 

[Rdslw]

Anything you do except gaming ? with that $ you can hit almost top tier parts. this is a bit overkill, and a bit over the budget. Feel free to lower specs of cpu,gpu depending of what you plan to do
I assume you want 1440p with 60 FPS:

PCPartPicker part list / Price breakdown by merchant

CPU: Intel - Core i5-8600K 3.6GHz 6-Core Processor (£203.99 @ Aria PC)
CPU Cooler: be quiet! - Dark Rock Pro 4 50.5 CFM CPU Cooler (£69.37 @ More Computers)
Motherboard: *MSI - Z370 KRAIT GAMING ATX LGA1151 Motherboard (£142.79 @ Alza)
Memory: *Team - Dark 16GB (2 x 8GB) DDR4-3000 Memory (£149.99 @ Overclockers.co.uk)
Storage: *Crucial - MX500 500GB 2.5" Solid State Drive (£97.90 @ Aria PC)
Storage: *Toshiba - 3TB 3.5" 7200RPM Internal Hard Drive (£63.02 @ CCL Computers)
Video Card: MSI - GeForce GTX 1070 Ti 8GB Video Card (£439.99 @ Overclockers.co.uk)
Case: Fractal Design - Meshify C TG ATX Mid Tower Case (£72.50 @ Aria PC)
Power Supply: *Corsair - RMx 650W 80+ Gold Certified Fully-Modular ATX Power Supply (£89.98 @ PC World Business)
Total: £1329.53
Prices include shipping, taxes, and discounts when available
*Lowest price parts chosen from parametric criteria
Generated by PCPartPicker 2018-06-12 08:22 BST+0100

you can take 8600 and gtz 1070 to make it ~1000 or change it to i7 if you wish more WORK pc