Jud, Mermaids is a group that supports teen and preteens who feel they are wrongly gendered. There have been some controversies surrounding them especially involving children as young as 4.
I'm not a fan of blue links, they are generally biased so Google can help you there.
TERF stands for Trans Exclusionary Radical Feminists or basically anyone who has doubts and questions about MtF transitioning.
That'll be us, you and me and others in this thread. It's used as a term of abuse. I wouldn't recommend Googling that tbh!
I would recommend, and I'm sure Rigby would too, looking at the Jenni Murray threads on Mumsnet at the moment.