Here in the States we do, indeed, adhere to a 250 kt below 10000' restriction. Houston TRACON (Dep/Arr controllers) did have a waiver to eliminate that restriction for a couple of years as a trial but they have since rescinded that and have restored the 250kt restriction.
Standard reason for having the speed held to 250kt is because of all the uncontrolled (i.e. general aviation) traffic below 10000'. I would think that even though there are not the numbers of light aircraft in the UK that there are here you still have a significant number clustered around your terminal areas and you would want to get the jets up and out of your lower airspace.
My company's SOPs require us to climb at BESEC (green dot for the 'bus) to 3000' agl before we accelerate to 250kts. Certain departure procedures like ATL and LAX want 250kts right away so we give them that.
Hopefully I will be flying into the UK in the next year or two after we sort out this merger so I will look forward to working with you gents in ATC. Until then I will keep myself up to speed as best I can via these discussions.