You could knock some flyers up Plod! fill in all the relevent details on how responsible you are and obviously you only want to shoot Vermin and get them sent out to the Landowners, there's been a good few flyers printed up on this site in the past if you can find them, also joining the BASC might help to show you have some cover...
I'm a bit lucky as in the past I would contact a landowner asking to fly birds at Rooks/Crows/Pigeons/Rabbits and general Vermin, I would normaly get a favourible response expecially if they come out to see the bird fly the quarry mentioned above, after a while