“Some sort of girly youth club ?”
The issue is to make it girls only, I think you need Equality Act exemption for this - ie to exclude boys. You can do this. I mean you can also say applications to be a church minister must be Christians etc. And you can say that a youth group is to be females only and give reasons like females underrepresented in certain domains in life, thrive in single sex environments for some things etc.. But it’s a faff.
After Girlguiding threw out both me and my daughters (aged 6 and 10 at the time), because I raised concerns about safeguarding and disability discrimination, I set up a youth group. I loved running Brownies, having started volunteering for them about 20 years ago, but when setting up my own youth group I couldn’t be bothered with the faff of making it girls only. I’m also not sure I could have fully justified it.
So if Girlguiding dies, which I hope it does because it appears that the Board will never admit that they’re wrong, or that their volunteers are anything other than stupid and couldn’t understand something as complex as their activity centres need refurbished, or different legalisation exists in countries where BGO operates, I don’t think any single sex entity will fill its place. Which is sad. What a legacy their Board will leave.