The overall Minister responsible for prisons is Rory Stewart, although I think for any individual complaint you would start with your local MP as Eddie suggests.
You have to start by writing to your own MP. If necessary the MP will pass on the question to the appropriate Minister. Ministers do not deal directly with ordinary members of the public.
the usual common law rule is that
you have to exhaust local "remedies" (ha!) before going to outside agencies. If you dont - all they say is 'fanx no fanx exhaust local remedies first"
[two exceptions spring to mind - GMC and their dob-in-a-doc campaign of 2000-2005 - and the roman dutch legal system in the dutch republics 1895-1900 - the "years of anarchy"]