You can't start maternity leave until 29 weeks so does that mean you have been signed off for a further 8, or you know you will be? If you will be unfit for any work throughout your pregnancy you don't need to start mat leave until 36 weeks.
I wouldn't suggest handing in your notice yet. If you stay employed throughout your maternity leave you will continue to accrue holiday and if you don't return, will be paid it out at the end of your leave.
Will you have been employed for 26 weeks by the 15th week before your due date? In which case you may be entitled to SMP rather than MA, provided your earnings are high enough, which as you've been off sick, they might not be if you're not getting much sick pay.
There's no advantage to handing your notice in so soon, so I would suggest not doing that, taking maternity leave, then if you still don't want to go back after maternity leave, give the appropriate amount of notice so that your employment terminates at the end of your maternity leave. Unless you get a job elsewhere in the meantime of course.
I am 21 weeks pregnant and I'm a few weeks away from completing my 6 month probationary period. I've been unhappy in the job and I don't wish to return after maternity leave. Added to which, I have been signed off by my GP for the last 5 weeks due to various pregnancy related issues and am unable to return to work.
So, I will not be returning to work before or after the baby is born. If I handed my notice in before my probationary period was up, would I be eligible for MA?