AK the way it works is your employer must automatically assume you won't be coming back until the end of your AML, unless you give them formal notice that you will be coming back earlier than that. It's fine to give an indication before you go off of what you think you might do, as you have done, but that's not at all binding, and officially unless you write giving 4 weeks notice that you want to come back early, then the position remains that your return date is the end of your AML.
You can imagine why this is - so many women change their mind about this once they have had their baby, as you have done! So asking women to confirm so early on what they will be doing is unworkable.
As far as your notice period is concerned, if you don't want to go back at all, you need to give notice in the same way as you normally would, i.e. 4 months before the end of your AML, so not for a while.
You say you don't want to mess them around, and you could give notice now if you are absolutely sure. But don't make any decisions about your actions based on messing them around - as I said, they have to be working on the assumption that you will not be coming back until the end of AML anyway, so if there is the slightest possibility that you might change your mind in the next few months, I would just let them know that you are planning to take your AML, and then resign at a later date if/when you are absolutely sure. You mustn't feel guilty about keeping your options open as long as possible, and as you are on 4 months notice anyway, even if you hand in your notice 4 months before the end of your AML, that still gives them plenty of time to sort out a permanent replacement.
Enjoy the rest of your mat leave anyway