It is a good question.
I can think of a few likely organisations to consider in possible order of increasing fees:
Public funding body such as National Audit Office
Independent specialist organisation such as ECRI
Then the private sector management consultants - Deloitte, PA Consulting, SECTA - the list is long. Another group in the private sector are law firms. Not sure if this comes as a surprise but there are a number of law practices that have people spacialising in PFI work on medical engineering services & equipment planning - if any of you have ever been to the Confederation's annual conference you'll see what I mean.