This is how I got my Great Aunt to be assessed. I was able to book an appt with her GP. (Thankfully, and very randomly, before we'd got power of attorney sorted she asked me to set it up so I had authorisation with her surgery, but they might be able to help if not.)
Anyway, I explained to the GP all of my concerns and upon looking at his computer could she was due a medication review. So we used that pretext to get her to the doctors. I took her for the appt and whilst there he then did the mini assessment. My Auntie probably wouldn't have listened to me, but her generation were brought up to hold doctors in high esteem and will listen to advice. For us that's worked. The problem is that everyone is different, and I think it's trial and error I'm afraid.