The only context in which you can be technically still employed but be refused access to your work tools is if you are suspended for a grave mistake. Abuse this at your own risk, as the employee isn't paid in the mean time and will very likely get tribunal-happy. Any other concept (paid leave, maternity leave, vacation leave, * leave) means you are still an employee of the company and the company is legally obligated to give you access to your work tools.

On leave, you are trusted as an employee to not do any work. Anything you do will be your own responsibility, or eventually the employer's responsibility to refuse any work you hand over. You are also not covered by the company's insurance on your leave. Your company cutting off access is a clear breach of contract.