How to greet when having a PhD panel interview?
Assuming you're referring to the proper addresses of faculty members at UK institutions, the general form is "Doctor" for faculty members who are Lecturers (known as "assistant professor" in some other countries) or Senior Lecturers (known as "associate professor" in some other countries) and "Professor" for faculty members who are Readers (varies between "associate professor" and "full professor" in other countries) or Professors (known as "Chairs" in some other  countries.  In some UK institutions, the North American standards have been adopted, so it's not completely uniform.

If you're uncertain as to the academic rank of the person, defaulting to "Doctor" will not likely offend.  If someone is introduced as "Professor", then use "Professor" in such cases.

In terms of matters of greeting not related to the proper address of the members, there are no formalities upon entering or leaving that you should be concerned about.  A general "Hello" / "Thank you.  Goodbye" or any variant are perfectly fine.