Hello you are absolutely right to raise a grievance about this. It amounts to a breach of trust and confidence and is also potentially a criminal offence contrary to s. 88 of the Postal Services Act 2000.
I would suggest that you say that if this is not rectified you will have to consider your options which may include resigning as an employee and claiming constructive dismissal and also reporting them for the offences under the Act.
You can also report the issue to the Royal Mail by calling 08457 740 740
Hi I'm really sorry but if you are not prepared to leave your job and you have already reported the incidents and the Police and Royal Mail are not going to take action it seems there is very little you can do. The only exception to this might be in the event that you can show that a financial loss has been suffered as a result of the actions in which case you may be able to consider a civil action against the employer but this seems to be unlikely on the basis of the facts that you have given me.
Please let me know if there is anything further you would like to know about this. If I have answered your question I would be grateful if you would take the time to rate my answer. Thank you and all the best.