I originally trained in Music, but now I'm disillusioned with the prospects and have no desire to teach it at secondary level.
I'm considering doing a Music/Philosophy (aesthetics) PhD and then applying to a PGCE philosophy programme. I only have a GCSE in Religious Studies, but believe that my PhD proposal will be accepted at a good institution. I'm less confident about the PhD though.

The PGCE is my plan b, but I want to make sure that it's viable before committing.

What is your career goal? It's hard to advise without knowing this. Is it to teach a specific subject at a specific level? I may have misunderstood what you are asking for advice about though. If it is to know whether you can do a PGCE in an unrelated field after a PhD, probably the best bet is to check the PGCE application criteria and maybe even contact some departments to get their take.