Starting freelance work during maternity leave

I’m currently employed with an agency and due to go on maternity leave in March. My income will only be supplemented through Maternity Allowance.

I’m considering building up my freelance work while I’m on maternity leave and I have a few questions:

- When should I reasonably communicate with my employer that I am intending to go freelance, so I’ll need to hand in my notice?
- Will this affect my MA and when do I need to inform HMRC / what can I earn while receiving MA?

Thank you for any advice in advance!