Reddit mentions of One Teacher in Ten: Gay and Lesbian Educators Tell Their Stories

Unfortunately, even today, it very much depends upon the school, so my advice for her would be to wait and feel out what this school is like. I also don't know what the laws are around sexual orientation and employment in England (especially if the employer is a religious organization), but recommend that she check them out.

If she determines that her job wouldn't be at risk if she were out at school, it's still totally her decision. One factor to put on the side of coming out is that there are pretty much guaranteed to be LGBTQ students at her school, and many of them probably feel very alone. An openly lesbian teacher could be a role model that gives them hope for their futures, and she might be someone they could turn to for support and advice. By being out, she could also help kids who aren't gay gain an understanding that gay/lesbian people are still people.

Even if she doesn't come out, I encourage her (and all teachers, gay or straight) to explicitly require tolerance in their classrooms, mention important LGBT historical figures, crack down on "gay" as an insult, etc.

*Stealth edit: Oh, I recommend One Teacher in Ten, a 1994 collection of essays from LGBT teachers, or the 2005 second edition of new essays. They are incredibly moving.