I know its easy when you have transport and can cover a lot of ground, but -
ask around, folk in the countryside know lots of people, especially who owns what.
Be careful about your approach though as you might be seen as someone who they do not want around. If you can help a busy farmer/landowner in any way, then offer to do so. Perhaps offer to pick some litter from his roadside hedges, bag it up and ask if he wants to burn it or should you take it away. Offer to watch out for preditors on his land at springtime, when the lambs are born. You could even offer to help at haymaking but its damned hard work!
They can be a very secular bunch but, be polite, be prepared for a put-down, be ready to come back another time if he is busy.
It will be especially difficult in hot,dry weather to get permission to light a fire - so take every precaution. And to the same end, invite the owner to inspect your site when you leave.
I find it esay to ask around, but then I'm living amongst them.
All the best though.
Ogri the trog