My personal opinion is that :-
If you are batting down a road at, say, 25mph in a 30 zone, on a bike, it may hold a driver up, it may not, but the majority here seem to have a downer on vehicle drivers.
So, if the limit is dropped to a 20mph zone (which I agree with in highly congested areas, outside schools, shopping areas etc.), do you think it's right for cars to be doing 20mph while you whizz past at 25mph ?
This brings to mind pot and kettle (the law applies to you, not me) and seems to me to be just a tad hypocritical.
Imagine, kid on way to school, sees car going at 20mph, thinks " I can get to the ped island before car gets here, so runs across road, only to get taken out by a cyclist doing 25-26mph that he didn't see overtaking the car.
I'm sure the fact that you are technically (if not legally) breaking the law, will really enamour the cycling fraternity to motorists.
BTW. I don't need a speed camera or sign to tell me to drive at 20mph, if I think 10mph is the safest speed to drive at, that's the speed I do (and yes, at 3am in the morning, when I'm on the M6 all alone I'll do 90mph, because it's safe and empty.