I think they're just Church of England so by default protestant, their website doesn't seem to mention a denomination and that's normally the default when people say they're a Christian in England. It's really not that cult like from what I've seen, I grew up in the church and honestly Grace and her family's religion seems normal for devout, practicing Christians in England. Her family is quite high up in their church - both her parents, Chloe and who I assume is Chloe's husband are all in senior positions - so they're naturally going to be more dedicated.