Having a NM bone scan is very much standard practice.
I received three letters on the same day booking me in for MRI, Biopsy, and NM bone scan. In that order spread over about two weeks.
I then went for each test in turn. I think they would have cancelled the subsequent tests if either the MRI or biopsy had came back negative.
So don't worry about the bone scan, the consultant does not think you have metastatic cancer, but he may as well check.
Unlike my trust where they book you in for all the tests and then cancel if not needed, your trust has booked you in for the bone scan after doing a biopsy.
If I were 64 year old with a PSA of 31 and I'd been booked in for a bone scan I would be assuming I had cancer. It is possible you will be getting a letter tomorrow cancelling an unneeded bone scan, but I wouldn't count on it.