[UK] Supervisor asked me to put different manager for my performance review, what do I do?

Hello, my supervisor and I don't get along well, but we try to make the best of it. Last year she gave me a weird performance review, writing that I lack skills on things I didn't have a chance to work on yet, that I lack emotional intelligence (because I cried after she yelled at me once). long story short, I talked to management and requested matrix supervision. In my institution, performance evaluations are part of permanent HR record, and I did not want to risk another year of her having unilateral control over it.

Management is aware of ongoing issues with this supervisor and agreed, so one of the managers became my matrix supervisor.

I sent my self-evaluation to both supervisors in the matrix structure over a month ago. My direct supervisor did not respond until my matrix supervisor (the manager) went on annual leave two weeks. She then kept pushing me to have a performance discussion bilaterally without the manager, as the deadline was approaching. I politely refused because according to organizational policies, I have a right to have a discussion with both my supervisors, and they both need to agree on my performance evaluation.

The manager will be back from leave on Monday. Now, we have a long weekend for Easter. Tonight, after work hours, before the long weekend, my direct supervisor sent me a message on WhatsApp telling me to change my direct supervisor to the manager in the system, so the manager can do my performance evaluation.

I don't have that authority to change my supervisory structure, that's a management level decision and needs to be changed in the system by HR. I'm fine with following the regulation of having the discussion with both supervisors, I never requested a supervisor change. I requested matrix supervision, which was implemented last year, although I wouldn't mind a change of supervisor.

How do I respond to this? I don't think this is a discussion for WhatsApp during a long holiday, so I'll wait until Monday. But I don't know how, what, where and to whom to respond to this. What should I do?