Need help to give practical test

Hello all, I have a test coming in 3 weeks and I am trying to find an instructor who might give me a refresher course (less important) and take me to the test on the day (main point).

I have my provisional DL, and an IDP and I thought I could hire a car to get my test done (which is not the case anymore). The instructors, here in Bristol, say if I have already booked a test then they can not help me (idk why?)

Can someone please suggest what should I do?

More context: My test is booked in Cardiff