How do I find a reputable functional doctor?

I'm at the end of my rope with ongoing, chronic health issues which are getting worse and I am getting no nearer to finding a way to manage or ease these issues .

So many people on the various worldwide support groups I follow seem to have received such good advice and treatment from functional doctors who seem to treat them holistically and piece symptoms together which is something the NHS won't do with me (I'm convinced my myriad of issues are connected).

Has anyone tried this and could anyone point me in the right direction if they've seen a functional doctor?