Try dressing not like a chav then?
Be polite, even if they say no. Just go back a couple month later and ask how they getting on with their rabbit problem.
Dont just knock and say "can i ferret on your land" ... have a chat, mention about noticing the amount of rabbits in an area easily veiwable (so they dont think your snooping around the land), give them info on what type of hunting you would like to do.
but dressing smartly helps too.... i would not even consider letting a chavy dresser tag along with me. experience says that most chavy dressers,,, are chavs!