It depends on the level, route and holding situation. If you were heading into a hold for a 10+ minute delay I'd expect you to reduce to minimum clean speed at about 50 miles out from the hold.
Below FL200, a lot of operators seem to fly at about 270Kts if there is little or no delay. Fly whatever speed suits you best, and if you want to change speed significantly don't forget to let us know.
Any particular airfield or route in the London TMA?