In the UK, can civilians outside of the employ of police, enforce offences that have not been decriminalised?

police cannot delegate their duties, obligations etc, therefore the legislature would have to pass law which allows them to do so or delegates some of the police jobs to relevant companies/agencies

Speeding is a special case.

With most crimes evidence can be accepted from any witnesses. So if you witness a theft you can give evidence in court and the thief can be convicted. All citizens also have a power of arrest, provided an offence HAS taken place (the police can arrest on suspicion that an offence has taken place).

The role of the police is to investigate the offence, collate the evidence, and present it to the CPS, who then decide whether to prosecute.

The issue with enforcing speeding is nothing to do with it being a criminal offence or not. Specific rules mean that evidence of speeding can only come from 2 or more police officers, or from a speed camera or gun that has been calibrated and used by a police officer. Even if 20 people witnessed a boy racer doing 60mph in a 30 zone it would be impossible to prosecute them for speeding.

The police can only delegate powers where that right has been granted by law. As far as I can see from the statutes you quote, only certain functions can be delegated, and I cannot see a way to delegate the enforcement of speeding to private companies.

the police have the authority to enforce the law they have access to the police data base.

to put that into the hands of a private company would not sit well with a lot of people