Never depend on one dictionary, because these bastions of accuracy aren't so consistent.
It seems to be that when used as slang (he's such a boy scout) it is lower case, but when use as a member of, it's upper case (I brought a cookie from a Boy Scout.)
Again:
You
look it up in the dictionary.
If you're specifically talking about the organization, it's upper case. If it's the adjectival allusion, it's lowercase. The link to the AHD actually specifies that.
Comparing British English and American dictionaries is daft. Pick one.
If you're using British English, use a British dictionary. If you're using American English, pick an American dictionary.
And, yes, you pick one dictionary and standardize on it. Doesn't really matter which one, but pick one. Consistency is your friend.
Ultimately, it's going to be a style guide decision, determined by your editor, if you're trade publishing, and you if you're self-publishing. They'll have house style guideliens, and a house dictionary.