Are the AGM minutes of an unincorporated body a legal document?
"Legal document" is not, as far as I am aware, a legally-defined category in English law. I would usually consider a legal document to be one having some legal effects (see https://en.wikipedia.org/wiki/Legal_instrument). For example, a document exercising a statutory right or a contractual document. It's possible that, in particular circumstances, such minutes could have a legal effect; but in many cases they would not, being simply a record of proceedings.