>>> He was interviewed, is that the same as a caution
No. I'm not sure that the possibility of a caution is relevant here anyway. If the guy was told that there were 6 months for the CPS to lay charges before a court then, clearly, he can't have been cautioned for the offence.
I can't help wondering here about whether the guy has been entirely truthful when relating his account of the events to his mother. It's highly unusual for someone to be arrested, and taken to a police station, for speeding alone. Speeding is usually dealt with by a fixed penalty notice which (in situations where the police have stopped a vehicle) is simply handed to a driver at the roadside. Even when the vehicle's speed is so excessive that court action is deemed necessary, the driver is normally sent a letter through the post, rather than being arrested.
https://www.gov.uk/speeding-penalties
Other relatively minor motoring offences are also generally dealt with in a similar manner:
https://www.gov.uk/stopped-by-police-while-driving-your-rights/minor-motoring-offences
For a driver to be arrested, there would normally have to be something else involved, such as a suspicion of driving under the influence of alcohol or drugs, having no tax and or insurance, etc. It all seems very odd to me!
However, if we assume for the moment that there really was nothing more than speeding involved, I suspect that a driver would need to be
substantially over the speed limit in order to get arrested. In such a case the police (together with the CPS) might need time to consider whether to simply issue a 'notice of intended prosecution' (which has an option to pay a fixed penalty and accept points on one's licence) or to go straight to a prosecution in court. Either way, the driver can expect something unpleasant to arrive in the post!