This thread has comparisons between private and NHS treatment, coming from a different direction than you, but the comparisons are still relevant.
https://community.prostatecanceruk.org/posts/t27019-Switching-from-Private-to-NHS-care
Even though I and others think the NHS is just as good as private. I would have that private consultation.
When my first PSA came back in the high 20s I asked my GP for a retest just to confirm, it came back more or less the same. I didn't want to start down the PCa route based on one sample which may have got mixed up with someone else's in the lab. So yes get a second opinion I think it will be the same as the NHS one but you need to be sure.
With T2 cancer there are private treatments which are not available on the NHS. But at T4 I think you are looking at hormone and/or chemo, there may be treatments the NHS won't fund but I suspect they would be very expensive.
I guess there are things like immunotherapy which is pretty cutting edge, but it would probably cost an absolute fortune and I don't think is proven.
Trials are an interesting idea. I think there was a thread on here about them, you have to think about possibly much worse side effects and that it may not work as well as proven treatment. So they may be better for people who are terminal rather than incurable.