What is the speed limit in European airspace?
Airspace regulations do not fall under the authority of EASA. Each country has their own set of rules published under supervision of their own National Aviation Authority in the AIP.

The origin of the 250 knots IAS limitation below 10000ft can be found in the ICAO airspace classes definitions. The ICAO airspace definitions include a 250 knots IAS limitation below 10000ft / FL100 in:


Class C: VFR
Class D, E, F and G: both VFR and IFR


That means that in class A and B there is no speed limitation below 10000ft / FL100. In class C, IFR has no speed limitation, but VFR has. In Europe you will find a lot of class A, B and C airspace below 10000ft. In the US, there is no class A airspace below 18000' feet. There is class B and C airspace below 10000ft, but the FAA basically put a blanket speed limitation of 250knots below 10000ft, even inside class B and C airspace. 

Also note that ICAO airspace classes are recommendations, countries have the authority to deviate from the ICAO recommendations as long as they publish these deviations. 

Section ENR (En-route) 1.4 of the AIP contains the ATS Airspace Classifications and states includes the deviations from the ICAO airspace definition.