Pre-built is often the way to go at the lower end of the market, custom building often only makes sense to those who want high end with specific requirements or enthusiasts who wouldn't consider pre-built. I've have a quick look around system builders as there isn't a profit margin built into each individual part in the same way as there has to be for a separate parts reseller.
//edit with that said this is a system that should work similar to one I specified the other day. Very basic but it would do the job.
Intel Celeron D 326 2.53GHz (LGA775) - Retail £27.99
(£32.89) £27.99
(£32.89)
Antec NSK4400 Mini Tower Case - 380W SmartPower PSU £42.99
(£50.51) £42.99
(£50.51)
Samsung SpinPoint P HD080HJ 80GB SATA-II 8MB Cache - OEM £25.99
(£30.54) £25.99
(£30.54)
Crucial 512MB (2x256MB) DDR2 PC2-4200C4 Dual Channel Kit (CT2KIT3264AA53E) £31.99
(£37.59) £31.99
(£37.59)
Liteon DVD-8900 16x DVD±RW/RAM (Black) - OEM £14.99
(£17.61) £14.99
(£17.61)
Abit IP-95 Micro ATX (Socket 775) PCI-Express DDR2 Motherboard £31.99
(£37.59) £31.99
(£37.59)
Sub Total : £175.94
Shipping cost assumes delivery to UK Mainland with:
City Link Parcel Next Day (Delivered Mon-Fri)
(This can be changed during checkout) Shipping : £9.95
Vat : £32.53
Total : £218.42