There is occasional chat about it on Bike Radar and I think some people have been cautioned (possibly mentioned somewhere in this thread but maybe other threads too)
[url]http://www.bikeradar.com/forums/viewtopic.php?t=12603098&start=600[/url]
I think if you look like you are purposefully breaking the speed limit then there is a chance they will pull you over
The thing is to do a competitive time your average speed needs to be very close to - and of course ideally above - the 20mph limit