Summer sales in London are on now, I noticed Selfridges and Gap (and bought a nice soft white leather belt for £10) on a trip up Oxford Street and have just received an email from Urban Outfitters to say that their sale has started (unfortunately it appears to be the worst sale in London).
EDIT 29/06/06: Sales are everywhere now, there is a list in the current issue of Time Out… I have just bought a shirt I had my eye on in Paul Smith that was reduced from £130 to £91.