Wait a bit for AMDs new platform. You could very well end up with an 8 core that essentially matches single thread performance of the i7 and i5 but with far better multi core performance.
Benefits with AMD "should" be better price overall for like for like OR a better system £ to £.
You'll not need to wait too long, the product code name is Ryzen or Rizen.
If you can't wait, you have SCAN over in Bolton. Top guys, one of the best for what they do and their staff are very good at speccing the system to your specifications.
I'd opt for an i5 none K chip. Make sure it's a 4/4 format rather than an 2/4 just on the offchance you go for a mini system that uses mobile parts.
Look for 16GB of ram. Two 8GB sticks clocked at 2400mhz should see you with a good few years of life and the facility to add two more without straining he memory controller on the CPU.
With i5 you are looking at 6 series or 7 series. Both take Z/H/B1x0 and Z/H/B2x0 chipsets. Save money and go for the 1x0 chipset with a 6 series chip, the performance difference will be about 5% max but a saving on i5 with decent spec board of probably around £80.
SSD in SATA format will be fine for what you require. You could go for pci-e with nVME support but day to day is pretty pointless.
I'd opt for an Samsung EVO 512GB although an 256GB would also be plenty for the most part.
Storage wise, 5900rpm green drives 2-8TB. Take your pick.
Decent PSU. Don't bother with multi rail. If a rail fails the PSU fails so who cares if its single rail. Having single rail also avoids overloading the rails power draw. A decent Antec, seasonic or Corsair unit will be fine. 550W will be plenty on the 12V rail as the system without a silly GPU will only draw circa 230W under load anyway.
GPU wise, something like an AMD 470 with 8GB of Abram will keep you going forever and cheaper options will also be fine (for the most part) but be sensible.
Cooling, a good case with good airflow. A couple of intake fans and an exhaust will do the trick. An reasonable air cooler or basic single rad all in one will also handle the none K i5 perfectly well so don't spend anymore Jan about £35 here. Options around the £20 mark will do just as good a job but may not be quite as quiet.
The above will allow for a good spec at sub £800 with OS. Changing odds and sods here and there will allow for he price to move and any one of the sales guys at SCAN, OcUK, PC Specialist, Cyberpower, chillblast or CCL should be able to recommend and act accordingly from the above.
Don't compromise on a single stick of ram. Don't compromise on a cheap nasty SSD. Don't compromise on a PSU. Everything else is pretty much fair game!!
Drop me a PM if you have any questions.