The only way to bottom this one out is to get the exact spec from PC world and build the same machine using the configure option on the website.
Price differnces can be due to
Motherboards cheapest £50 v dearest £100
Soundcards onboard or sep card
Not all 17 TFT's are the same quality
There are some good deals at PC world but I have always found better spec machines from Evesham,the ex Carrera and the best buys in PC mags etc
Prob the bext bet is to look at the recommended machines on PC advisor and pick their best buy at the price point you choose.