In the rural UK area where I live farmers will often be happy to have you shoot them with an airgun, if you seek permission. Nice (if stewed) cheap meat.
Farmers in England and Wales have a legal obligation to remove rabbits. You are doing them a favor by volunteering to do so.
Goodness, I don't imagine a regular airgun is humane for rabbits? The FAC limit is 12ftlb?
We used to use a tight .410 which I think humanely dispatched them but I think an air gun would be much better for the meat.