In the UK there's a law covering this specific case. Theft is deigned "taking wi...

In the UK there's a law covering this specific case. Theft is deigned "taking with the intent to permanently deprive", so joyriders were defending their actions by returning the vehicle after just this.

Therefore, there's now an offence for this: Taking Without Consent: https://www.cps.gov.uk/legal-guidance/theft-act-offences