Getting back to the original post...20mph speed limit, yes it will apply to bikes, it will apply to any vehicle that is legally allowed on the road.
Rules....Made for the guidence of fools, and the obedience of wise men......
I would have thought so yes, but there seems to be differences of opinion on whether UK road speed limits apply to bicycles. As far as I'm aware, they apply to everyone on the road, including cyclists. However the problem is whether a cyclist is capable of knowing they're going over the 20mph limit. But as has been stated, ignorance is no defence under the law.
If these 20mph limits were fully supported by central government and actively enforced and you got done by a police officer who spotted you, then the lack of any speedometer wouldn't be any defence. This 20mph law then, would technically require cyclists capable of doing 20mph to have a speedometer fitted in order to comply with the law.
However this discussion is purely theoretical because:
1) the 20mph by-law some Council's are trying to adopt, are not likely to be practically enforceable by local police authorities who say they're not willing to police them (maybe the police can foresee the can of worms with cyclists, this is likely to open)
2) if the police are not willing to enforce them, speed cameras will have to be used and cyclists won't be able to be prosecuted with no number plates.
Looking at the challenges of introducing a 20mph speed limit, particularly the costs in a time when Council's are under increasing financial pressures..it seems likely very few Council's will be willing to adopt these 20mph limits without the full backing of central government and the police behind them.. its more likely they'll go for a simple low-tech solution to the problem, like speed bumps, or adding pedestrian crossings in these dangerous road spots in built-up areas.
The Council's may just add the 20mph speed signs and hope the majority of motorists will observe them without any camera technology, such is the case with other speed limits pretty much. And then if that's not the case, then the Council will have to look at different enforcement options.