29 weeks earliest (Although I can go at 26 weeks unpaid for 3 weeks).
If you have annual leave (which you will also accumulate on mat leave) you can tab this on the front (I am so I can get out sooner)
You don't need the Matb form to tell them when you want to go, just have to hand it in at 25 weeks as proof of pregnancy. Best to hang on for it though, don;t feel bad for only giving them 4 weeks notice, it is only the same as if you were resigning.