Hey there,
We were married by a humanist in Scotland. You have to get a license from the office, but the officiant is registered, so you can get married where ever you like. You don't have to find a place that is licensed, so you can get married up in a tree if it pleases you and them. I think there are a couple lines you must say to be legally married, my wedding seems like a lifetime ago, so would have to go back and check. And we have one of their wedding info books if you would like it.