If you are in Cat A airspace where all traffic is separated by ATC then there is no speed limit below FL100. The 250kts below FL100 is to aid the "see and avoid" principle.
The exceptions to this are when a speed limit is published on the plates. Manchester, as an example, limit departing speed to 250kts below 100, but the arrival speeds are at fixed points, not altitudes. eg 5nm north of trent vor.
There is a good CAA AIC available on the web that explains it all.