First, she was Welsh, not English, so I should probably type her name as Gwenfrewi, though I'm not sure how to pronounce that. Some 16th century cardinal (Baronius) tried to claim her for the English, but he needn't have bothered -- the sixteenth century was a great time for creating English saints and martyrs.
Not that Gwenfrewi's' story is less compelling than say Thomas More. It's just more personal and less political in the sense of national government and foreign relations.
Gwenfrewi was the only child of a noble family. Unlike most women of her time, she was literate -- like many, she was dedicated to her faith rather than service to a husband. When a local chieftain courted her, she told him she was already engaged to another and sought refuge from his anger in her uncle's oratory (private chapel). The would-be husband rode up and cut her head off before she could run into the building.
Uncle Beuno was a holy man, himself destined for sainthood, but even a saint can lose his temper when faced with such sinful cruelty. He cursed the husband, who melted like wax. Then he put Gwenfrewi's head back on her body and prayed; her life was restored, leaving only a thin scar. As a bonus, water burst from the ground where her head had fallen, creating Holywell, the most popular of the Celtic holy spring sites. A mining company unintentionally diverted water from it in the early twentieth century, but another underground source was identified, so the water still springs today.
As for Gwenfrewi, Uncle Beuno gave her his oratory, and there she founded an abbey. She served as an abbess for fifteen years before taking the next big step toward sainthood.