A company that wipes an employee owned system without the employee being informed of and signed up to a policy authorising the company this access and modification over employee's private property would, in the UK be committing a Computer Misuse Act section 3 offence. That is a criminal, not a civil matter. If the company gets this wrong and the employee or ex employee complains to the police, the company decision maker causing this to occur could theoretically end up in jail.
As a UK company email administrator, if you were to implement such a policy without having it in writing from the pointy haired boss making this policy, it could be you that ends up in jail. Kind of situation where the administrator or PHB needs to be forewarned.