Helow,
The only reason you're boss needs to know that you're preggie now is because of health and safety, to look after you. When you receive you're MATB from you're midwife around 25wks you officialy let you're place of work know that you are preggie. Make sure that you're boss takes time to sit with you go through a risk assessment for pregnant women, what you can do and now cannot do eg. heavy lifting and stretching.
Only with the MATB do you let you're boss know when you are thinking of starting maternity leave and thinking of coming back. But if you want more time off, all you need to do is give him 28 days notice that you intent to take more off, and 28 days before you are due to go back you also have to write a letter to give you're date of resuming work. You also have the right to ask for part time hours.