I don't think it's a surprise to see a papal bull being used as just writing material at that date; the Papacy was disputed, and widely seen as a nakedly political tool, and thus lost much of its glamour.

I suspect that the last verse runs "have her at MY will", it's no fainting detatched love but a good mediaeval healthy desire. It's not at all uncommon at that date for secular love to be used as a metaphor for the spiritual, but I'm not sure this is one of them. It would be surprising to find a cult of St. Brigit in England- the English church rather looked down on their Irish colleagues, as evidenced by Gerald of Wales' story about the exchange between English and Irish bishops concerning the lack of Irish martyrs. The cult did travel ouside Ireland, but I believe mostly to areas influenced by Irish missionaries,   like France and Germany.