If you were'n't stopped and spoken to by the Police at the time than you are extremely unlikely to hear anything more about this. Video evidence from an in car camera can only be used as a back up to evidence from an Officer. The video camera itself is not a Home Office approved device in the same way as a Gatso or laser camera, so cannot be relied on to to prove speeding as the sole evidence in the case, and ofitself it cannot be used to prove careless driving.
They have your reg number but cannot prove identity of the driver from a foward facing in car video. For undertaking the relevant charge would usually be careless or inconsiderate driving. For that the Police must issue a NOTICE OF INTENDED PROSECUTION Under schedule 1 of the Road Traffic Offenders Act 1988 or summons fro the offence within 14 days of the offence occurring.
In all the circumstances the reality is that the cops wanted to give you a fright , but either were being called to something more important or when they lost you decided they couldn't be bothered.
Sit tight but I strongly suspect YOUR FRIEND'S? been lucky this time.