The policeman took your speedo as a mitigating circumstance and was obviously in a good mood/being human for once , so you didn't get charge with the offence in the first place. However he would have had every right to charge you even with it sticking - and then there is no defence.

As this was a speed camera, the Notice of Intent to Prosecute will be automatic, so no defence . Its a shit but it applies to everyone so its fair a suppose