I'm sorry you have such a lot on your plate.
Generally GPs are compassionate but a GP won't just write you a fit note for 6 months and your employer would likely want to maintain regular contact with you whilst off sick to understand how they can support you to return. You would be subject to sickness procedures. That might mean weekly or fortnightly phone calls to check in with you and get updates or might involve HR visiting you at home.
From your employer's perspective it might cause concern if you present a fit note once your maternity pay ends and then immediately present as fit to work once your sick pay ends.
What's your employer's absence policy for long term sick leave? In terms of absence management and moving to dismissal.
How were you originally planning to manage the return to work? Can any of that plan still be used in some way? You mention wanting to breastfeed until your son is 1, so that's not new (or an illness of yours) - how were you planning to manage that originally?
Going back to work after an absence of nearly a year and a half will be even more difficult. What's your plan for yourself in terms of how you get yourself to a place where that will be manageable? What support will you need to adjust and settle back in? The longer you are away the more nervous you are going to feel when it is time to return, so you should factor this into your thinking.
Are you receiving any support for yourself in caring for your parent?
www.mind.org.uk/information-support/helping-someone-else/supporting-someone-who-feels-suicidal/supporting-yourself/
Is there any support your employer could provide to help you manage this transition? Do you have access to therapy? Would an occupational health referral help? (If you're going to be off long term sick they may initiate that process anyway to obtain a medical report.)
You should be able to self refer through IAPT for free basic counselling or CBT on the NHS. If there is a charity for your parent's condition, they may also have helplines, support services or signposting to support.
https://www.nhs.uk/mental-health/social-care-and-your-rights/how-to-access-mental-health-services/
https://www.equalityhumanrights.com/en/managing-pregnancy-and-maternity-workplace/faqs-employees/returning-work-illness-end-maternity-leave