set some live traps for mice and let the dog have at them in a controlled environment, then work up to rats. then go clean out old barns full of hay/straw bales. as you push stuff around the little buggers will take off running and the dog will get the chance to do it for real.
Pretty much what pops said. In my parents farm we had working vermin dogs. We got their confidents up by letting her at trapped rodents. For example. We would find a gopher in a long pipe. Maddy would be waiting on the other side we would tip the pipe up and push the gopher out. Maddy would get it as soon as it came out.
With maddy though she had so much prey drive we really didn't have to teach her anything.
Oh we're past that point - she has a half dozen confirmed rodent kills (plus a couple rabbits and birds) at this point and competes in barn hunt successfully. I'm more curious about how you go from casual, random hunting to more serious, prolonged work.
Well that sounds simple enough! I wish there were more resources - a friend of mine has done some casual clearing of an auto parts shop and a lawn care business' storage area but she does it just for fun and doesn't charge - I'd like to see if I could turn this into a legit business, with a focus on local small farm settings since I know voles are especially hard to deal with for organic farmers (and that would reduce the risk of running across poison).
i'd hit the local feed mills and farm stores as you'll be less out there where whackos can try to take advantage of you. just a simple "i have terriers and I'm looking for a place for them to hunt and kill rats" with your number attached. the feed mills might even let you try your hand there and then give you references. another thing to help get you ready, join a british forum called the hunting life. lots of good knowledge in their terrier section on all aspects of terrier work.