Spartacus
It IS good value given it would cost you or I 650 to buy the parts. The Spartacus (at 599) was number 1 on PC Advisor's August 2014 list of budget PCs.
http://www.pcadvisor.co.uk/reviews/desktop-pc/3539289/chillblast-fusion-spartacus-budget-gaming-pc-review/
The cooler should be okay but monitor the temps. An H55 would cost the same as the chip, which would be nuts.
It is NOT worth getting different thermal paste. The only time I use it out of a tube is when I've taken off the cooler.
I would not trust that PSU, see "the power supply unit tier list" on tom's hardware. FSP premium isn't on the list but most FSPs are fine. The +25 corsair is a safe bet.
http://www.tomshardware.co.uk/answers/id-1804779/power-supply-unit-tier-list.html
Be aware that the case is very cheap (I can't find a review to say whether it's okay). Given that I can't find it being slated anywhere the AvP may do fine. Any of the other case options are larger.
Also be aware that OEM windows is tied to a single motherboard.
On Chillblast, I don't know where they're getting their 'from' prices unless they're achieved by removing the operating system, mouse/keyboard etc, since as soon as you click "customise or buy" it reads 675.99. With a blueray/DVD combo, the 600W Corsair PSU and their cheapest speakers that's £757.58.
Other Places
Also check out systems on:
Scan http://www.scan.co.uk/3xs
Aria http://www.aria.co.uk/Systems
Yoyotech http://www.yoyotech.co.uk/systems
Overclockers http://www.overclockers.co.uk/LP/systems2.php
CCL http://www.cclonline.com/
Those likely wont include a monitor (IPS for gaming) keyboard or mouse but you can get comparable monitor for around 100 (pcpartpicker is good for finding the best price on brand name computer parts). The keyboard and mouse in the deal are 5 and 7 respectively.
https://uk.pcpartpicker.com/parts/monitor/#W=2200,2400&sort=a10
Do try to get a quad core (i5) if you can.
As an i5 alternative, which will cost £100 more:
http://www.scan.co.uk/3xs/configurator/cheap-intel-gaming-pc-next-day-delivery-uk-g30i
Intel Core I5 4460
1tb Seagate Barracuda 7200 RPM
8gb Corsair Vengeance LP 1600Mhz 9-9-9-24
EVGA GTX 960 SSC 2gb
Windows 7 or 8
Corsair 550W PSU
Asus H81M-PLUS
Corsair Carbide 200R
Pioneer BDC-207DBK - 8x Blu-ray Reader, 16x DVD Reader & Writer
£725.68 inclusive of blueray upgrade, monitor/mouse/keyboard/speakers not included. As Robyn said the i5 4460 is the smart choice for performance to price. The case and RAM are both excellent, and that's the best selling version of the GTX960. Total ~£850.
The only drawback there is that there's no choice of motherboard and it only has 2 RAM slots. Since changing the motherboard is a huge PITA I'd go for the full 16 gig now. You are however looking at ~910 by the time you've added monitor etc.
Freshtech
Never used them (only came across them today!) and their customisation options are extremely limited, but their prices are great.
If you're happy to add the blueray yourself (or get one of these friends to do it for you) check them out. Allow £40 for blueray read only, or £55 for a writer. Advantage - 2 disk drives for if one fails! Disadvantage - akin to Scan the only offer one motherboard with just two RAM DIMMs. Disadvantage - the RAM isn't named.
http://uk.pcpartpicker.com/parts/optical-drive/#t=2&sort=a9
£590 ~ Sparticus
Intel Core I5 4460
1tb Seagate Barracuda 7200 RPM
8gb 1600Mhz
GTX 960 2gb
Windows 7 or 8
Corsair CX600
Gigabyte H81M-S2H
Choice of cases £619-£644. (clicky)
£685 ~ Scan Machine
Intel Core I5 4460
240gb Kingston V300 SSD
8gb 1600Mhz
GTX 960 2gb
Corsair CX600
Windows 7 or 8
Gigabyte H81M-S2H
Choice of cases £656-£680. (clicky)
OR
Intel Core I5 4460
1tb Seagate Barracuda
16gb 1600Mhz
GTX 960 2gb
Corsair CX600
Windows 7 or 8
Gigabyte H81M-S2H
Choice of cases £694-£718. (clicky))
£745 ~ Scan Machine with 16GB
Intel Core I5 4460
1tb Seagate Barracuda
8gb 1600Mhz
GTX 970 2gb
Corsair CX600
Windows 7 or 8
Gigabyte H81M-S2H
Choice of cases £720-£744. (clicky)
OR
Intel Core I5 4460
1tb Seagate Barracuda AND 120gb Kingston V300 SSD
16gb 1600Mhz
GTX 960 2gb
Corsair CX600
Windows 7 or 8
Gigabyte H81M-S2H
Choice of cases £738-762. (clicky)
Of the cases I'd look for something that comes with both intake fan and exhaust as well as a front USB 3.0.
CCL
They're cheaper on ebay than on their own site by a long way.
Intel Core I5 4460
2tb Seagate Barracuda AND 120gb Kingston V300 SSD
8gb Kingston 1600Mhz
Asus Strix GTX 960 2gb
Cooler Master B500 (needs research as too new to be on the tier chart)
Windows 7 or 8
Asus Z97-P (4 DIMMS)
Wireless card
http://www.ebay.co.uk/itm/CCL-Pro-Gaming-PC-3-4GHz-Intel-Quad-Core-i5-4460-8GB-2TB-WiFi-2GB-GTX-960-/381070347823?pt=LH_DefaultDomain_3&hash=item58b9900e2f
615
Win 8 = 693
Win 8 + 16GB = 749
Blueray + Monitor + mouse/keyboard
Extra RAM can be added at a later date since there are three free slots. Just be aware that it comes with 18gb which isn't quite as good as 24. Dual channel RAM is better added in pairs. 2* something gives more than double the performance of one of them.
Other useful links
http://www.tomshardware.co.uk/gaming-graphics-card-review,review-32899.html
http://www.tomshardware.co.uk/gaming-cpu-review-overclock,review-32901.html
https://www.cpubenchmark.net/high_end_cpus.html
http://www.videocardbenchmark.net/high_end_gpus.html