What do you doubt? That you could be convicted of speeding on the basis of a police speed gun or camera? Do you think that not having a speedo is some kind of magic get-out clause?
As a matter of fact I do have evidence of a conviction under these circumstances.
The world's first speeding ticket went to Walter Arnold who drove a horseless carriage at 8mph through Paddock Wood, Kent on 28 January 1896.
https://www.historic-uk.com/HistoryUK/HistoryofBritain/Walter-Arnold-Worlds-First-Speeding-Ticket/ He was fined 1 shilling for speeding.
The first motor car speedometer was introduced in 1901
https://www.motorhowto.com/how-did-old-cars-measure-speed/
So Mr Arnold was convicted of speeding some 5 years before the invention of the car speedometer. I think that counts
In fact, speedometers in cars weren't mandatory in the UK until 1937, two years after the introduction of 30mph limits in urban areas.
https://www.gov.uk/government/publi...-safety-the-highway-code-and-the-driving-test
Now I've never been done for speeding, so I could be wrong, but I think pretty much all speeding convictions are done on the basis of external measuring equipment, and no reference is made to the car's speedo.
Of course, if you want a helping hand in ensuring that you stay within the speed limit you are free to fit a speedo to your bike. Just as you should have a speedo in your car if you want it to pass its MoT. But not having one won't save you from a ticket if there is evidence that you were speeding.
