So, if you havn't had your baby yet, there should be no problem. Just tell them your new plans. They can't hold you to anything until 2 months before the end of mat leave.
You're legally entitled to 12 months mat leave. Just know that for the first 26 weeks your employer must keep your job open for you to return to. If you take Additional Maternity Leave (the remaining 26 weeks, up to 52 weeks total), your employer must try to offer you the same job back, but if that's not reasonably practical, they must offer you a suitable alternative role with the same or better terms and conditions.
As a manager, dealing with maternity cover is never a bad as the expectant mums think its going to be for me!
As a mum, I'd encourage you to take all the time you can afford, and want to. I love my career, but DS has just taken his GCSEs and it only feels like I went back to work from mat leave a few months ago. It's never been more true that the days are long, but the years are short.
Good luck with your mat leave, your birth, your baby.