LJ, in britain there are certain circumstances where the 250kts speed limit below FL100 can be ignored. For example: in class A and B airspace, where atc approve high speed in class d and e airspace. This information is taken from the ATC bible - Manual of Air Traffic Service Part 1. It is a whole page of restrictions and criteria but the above is why some times you can get "high speed approved" and other times not - hope this helps. However as a pilot i cannot believe all this is not written down somewhere for you?
[This message has been edited by information_alpha (edited 10 April 2001).]