OK, I would go for this:
PCPartPicker part list:
http://uk.pcpartpicker.com/p/pp9fLk
Price breakdown by merchant: http://uk.pcpartpicker.com/p/pp9fLk/by_merchant/
CPU: Intel Core i5-4460 3.2GHz Quad-Core Processor (£141.19 @ More Computers)
Motherboard: MSI H97 PC MATE ATX LGA1150 Motherboard (£61.40 @ CCL Computers)
Memory: Corsair Vengeance 8GB (2 x 4GB) DDR3-1600 Memory (£37.24 @ Ebuyer)
Storage: Samsung 850 EVO-Series 250GB 2.5" Solid State Drive (£72.37 @ Amazon UK)
Video Card: EVGA GeForce GTX 970 4GB SSC ACX 2.0 Video Card (£263.99 @ Aria PC)
Power Supply: SeaSonic G 550W 80+ Gold Certified Semi-Modular ATX Power Supply (£75.09 @ Ebuyer)
Total: £651.28
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-08-04 10:23 BST+0100
- That CPU is the best gaming CPU for £140 hands down, there's a Toms article called best gaming CPU's for the money, if you want to verify that.
- 8GB RAM is more than enough for games and 1600Mhz is the fastest RAM supported by that CPU.
- Nothing fancy bout the motherboard, just a good price and has everything you need. USB3, SATA 3 etc.
- Went for SSD only since you don't seem to need much storage. If you need more, you could up it to 500GB or add a hard drive. Samsung is a very reliable brand for SSD's, I've had one for years now.
- That graphics card is super overclocked out of the box so it is around 20% faster than stock. EVGA also tend to be quite good. The GTX 970 is also a great price/performance option, not far behind the 980 for a lot less cash. Should destroy any game at 1080P and does well at 1440P as well. Google GTX 970 gaming benchmarks to have a look if you want. Generally 30FPS+ is playable.
- Only the best will do for the PSU in my opinion, read a review of any Seasonic PSU and you will see they're the best in the business. Not cheap but you're basically buying reliability here so not a good place to skimp. EDIT: Also you only need like 350-400W+ for this build so 550W is more than enough power
- Didn't include a case because you already have one and they are very much personal preference. Any mid tower should be fine, just look at some reviews and see which one you like the look of most. The Fractal Design Define R5 is a good understated one, Coolermaster HAF 922 is more of a flashy high air flow case. NZXT Phantom 410 looks pretty cool as well. Antec Three Hundred is a good cheap case.
EDIT: If i was to buy a mid tower now, I'd probably get a nice Aluminium one from Lian Li or Silverstone, like this:
http://www.cclonline.com/product/178839/PC-A55A/Cases/Lian-Li-PC-A55A-Aluminium-Midi-Tower-Case-Silver/CAS2195/?gclid=CIz16MWQj8cCFQX3wgodGGQOyQ