Bother, I knew I was in danger of starting that old chestnut off again, and it wasn't the aim of my post.
Categorically, bikes are not subject to road speed limits. It's nothing to do with speedometers. It's because Parliament, for whatever reason, chose to frame a law that says you may not exceed the designated speed limit in (or on) a
motor vehicle. A bike is not a motor vehicle, so the law does not apply. You can see this throughout the
relevant section of the RTRA.
Parks, sea fronts and other public areas are governed by bye-laws, as opposed to road laws. If the Council makes a bye-law that bikes may not exceed a certain speed, that is the law in that park (whether the bike has a speedometer or not).
Of course, there are laws about reckless/dangerous cycling, and exceeding the limit for motor vehicles might be seen as evidence of recklessness, so you could still get prosecuted, especially if an accident ensued - but not specifically for exceeding the limit on the road.
Now, back to Rule 168...