You can get your MATB1 form from your midwife when you're more than 20 weeks - it has to be done in person. Then you give the MATB1 to your employer, who will give you an SMP1 form explaining why you're not eligible. You need SMP1 to claim Maternity Allowance, which you can do from 26 weeks onwards regardless of when you want your maternity leave to actually start.