The employer is allowed to make reasonable adjustments to your employment provided it does not fundamentally change the job or your status or income or location.
I appreciate there is probably a change of status here.
You would be entitled to raise a grievance with the employer for unreasonably changing the terms of your contract. They have inserted another rung in the promotion ladder.
However the bad news is that if your employer will simply not play ball and they simply tell you that’s how it is like it or leave it, your only recourse is to take them to a tribunal for breach of contract. You cannot beat them with a stick until they move you upwards or change the reporting structure.
If they have inserted this extra layer across the board and not singled you out, then it may be a necessary operational change.
If you think they’ve done it to make you leave or do some way of bullying you, then you have a claim for constructive dismissal if you feel that you no longer wish to work there as a result of this change.
You don’t have a claim for constructive dismissal if you have not worked at the employer for more than 2 years.
As there seem to be at least 2 of you in this situation, then it is possible they could justify this as being necessary for operational reasons and that the only effect on you would be the promotion ladder having an extra rung. Whether that is important to you are not would really depend on the realistic likelihood that you would ever climb to the top of the ladder or whether the extra rung does make that big a difference.