Despite your previous experiences, im still going to recommend BUILD IT.
1) Post ALL components on this website BEFORE you buy anything! If any parts are incompatible, or can be found cheaper elsewhere, or better alternatives are avilable, people on TOMs will tell you and help you.
2) NewEgg do a very very good 3 part video series showing how to build a system. Its done step by step in real time so literally all you need to do is follow it.
3) You WILL save a considerable amount by building.
On a £660 budget you could easily fit a 560Ti and an I5-2500K into that budget.
Heres a build with a similar budget i did for a friend, NOTE, this build can now be done a fair bit cheaper (this was written up a few weeks ago)
i5-2500k £161 (Ebuyer)
CPU cooler Arctic Cooling Freezer 7 Pro rev2 £15 (not necessarily needed, depends if you want to Overclock or not) (Ebuyer)
Motherboard Gigabyte GA-Z68X-UD3H-B3 Z68 £128 (SCAN)
RAM Corsair Vengeance 8GB £49 (SCAN)
PSU 500w Corsair Builder Series £45 (SCAN or Ebuyer)
GPU Asus 1GB GeForce GTX 560 Ti DirectCU II £173 (SCAN)
Optical Drive LiteOn iHAS224 24x DVD±RW £15 (Ebuyer)
HDD Samsung HD103SJ Spinpoint F3 £40 (SCAN)
Total: £626 VAT included.
with current prices that build is more like £580 ish maybe even less.
FINALLY - if you really really dont want to build, i suggest checking out a website called PC Specialist http://www.pcspecialist.co.uk/