Need help to build a gaming pc /w Sapphire R9 290X Vapour-X Tri-X

What will you be doing with this PC? Be as specific as possible, and include specific games or programs you will be using.

I will be playing games, mainly FPS / Open World type, like CS:GO and PUBG.

What is your maximum budget before rebates/shipping/taxes?

£400

When do you plan on building/buying the PC? Note: beyond a week or two from today means any build you receive will be out of date when you want to buy.

Next week, starting 24/12!

What, exactly, do you need included in the budget? (Tower/OS/monitor/keyboard/mouse/etc)

CPU, motherboard, PSU, case/tower, RAM and SSD. (non-stock cooler if needed).

Which country (and state/province) will you be purchasing the parts in? If you're in US, do you have access to a Microcenter location?

UK, west midlands

If reusing any parts (including monitor(s)/keyboard/mouse/etc), what parts will you be reusing? Brands and models are appreciated.

Sapphire R9 290X Vapour-X Tri-X, 2 TB Western Digital HDD

Will you be overclocking? If yes, are you interested in overclocking right away, or down the line? CPU and/or GPU?

I want to be overclocking CPU further down the line, but if this affects the cost massively then I do not need a system able to overclock.

Are there any specific features or items you want/need in the build? (ex: SSD, large amount of storage or a RAID setup, CUDA or OpenCL support, etc)

The best performance possible please! :)

Do you have any specific case preferences (Size like ITX/microATX/mid-tower/full-tower, styles, colors, window or not, LED lighting, etc), or a particular color theme preference for the components?

Since my GPU is blue, the theme of the build should be blue... but it is not neccesary!

Do you need a copy of Windows included in the budget? If you do need one included, do you have a preference? No, I do not need windows!

Extra info or particulars: Thank you in advance!