Interesting, I left 4 years ago and we were always told it was a legislative edict that ATC did not have the authority to permit for normal operations as an earlier poster stated.
Of course emergencies, medical cases etc no problems if declared and requested.
Hopefully things have loosened up.
Another curiosity was the 250kt was the maximum, having done and observed some JCAB checkrides if you were in unstable air and fluctuated anything above 250kts was "you broke the law" stuff and would bring heavy consideration as to whether you passed or failed the checkride.
Cheers.