Many thanks for your patience, I am pleased to be able to continue assisting with your query now. Firstly, you should argue that as the acting policy only allows 6 months maximum and yours was basically extended 5 times, it has created a precedent that this is not just an acting up placement but potentially transformed into a permanent one due to the length of time you have been in it and the extensions that have happened contrary to policy.
However, no matter what you raise with the employer about it, they do not have to agree with it and the only risk is that there is nothing definitive in law that would give you the rights you are after – it is mainly an argument based on reasonableness and fairness, but nothing specific in black and white.
Ion that case, assuming the employer does not agree with you, you may have to consider challenging them via other means, which would likely require you to resign and claim constructive dismissal against them to try and get compensation.
It is worth mentioning that there is a possible alternative solution, where the employer is approached on a 'without prejudice' basis (i.e. off the record) to try and discuss the possibility of leaving under a settlement agreement. Under such an agreement, the employee gets compensated for leaving the company with no fuss and in return promises not to make any claims against the employer in the future. It is essentially a clean break, where both parties move on without the need for going to tribunal. However, it is an entirely voluntary process and the employer does not have to participate in such negotiations or agree to anything. It just means that these discussions cannot be brought up in any subsequent tribunal claim and prejudice either party. So there is nothing to lose by raising this possibility with the employer as the worst outcome is they say no, whereas if successful it can mean being allowed to leave in accordance with any pre-agreed terms, such as with compensation and an agreed reference.
Hopefully, I have answered your query in a way that is simple and easy to understand. If anything remains unclear, I will be more than happy to clarify it for you. In the meantime, thank you once again for using our services.