Baby #2 on route

I’m currently pregnant with baby no 2 , I’ve just come to the end of my mat leave with my first child and have returned to work.

I will be due in September but would like to start my mat leave this time round in August.

I know August is always a difficult time at my workplace due to having to take annual leave for school holidays etc so I’m wondering if I put in a letter to my manger to my manager stating I would like to start my mat leave to start in August would she be able to decline this or is that not legal?