Probably buy from the wholesaler, or depending on their size / what they offer, may buy in bulk from internet / direct from business suppliers.
Ebuyer.com is one of the cheapest agreed. Also try overclockers.co.uk, and scan.co.uk.
Your going to be paying more for general shops like PC worl / Currys etc. Although the lower end systems, they can crank out for cheap (The ones including monitor OS system etc for £300) but as you move up the quality bracket, they generally add a fair bit ontop. Best browsing the deals, especially 'buy of the week' 'buy of the day' etc, to pick up things at good prices.