camden 20 mph

^this is sort of true. Bikes have no speedos so cannot be expected to stick to a speed.
That's incorrect. Legislation defines what vehicles / carriages that speed limits apply to. It is not specified that bicycles are vehicles / motor vehicles for the purpose of speed limits*.

The fact that bicycles do not have speedometers is irrelevant.

• In some Royal Parks, this is a little more ambiguous, and as yet untested.
  
  
  

Speed limits only apply to vehicles, bikes aren't consider ones.

Pretty much this.