This is a starting point.
NATS | AIS - Home
EDIT:
That link doen't seem to work. Try going to AIS.ORG.UK, then go to
IAIP -->> Aerodrome Index - Specific
If there are specific requirements regarding joins it will be in section 2.22, Flight Procedures. If you look at the AIP pages for Duxford it states 'there are no overhead joins at Duxford'.
You might have to be a bit more specific regarding the field your question is about. It might be more of a 'rules of the air' or 'airmanship' issue than an AIP issue.
Ultimately, at an uncontrolled airfield they cannot (by definition) tell you to fly around the circuit in a particular manner. But it does help everybody if you do it the way they ask you to. It possibly also means you're welcome to visit again should you wish to.