Phrases like 'tell her where to get off' and 'tell her where to stick her two days' seem a bit unnecessary to me tbh.
Your GP has given advice that you should be signed off altogether.
You have said to your boss you would prefer to keep working and have suggested working at home 2 of your 4 days a week.
Your boss has very sensibly decided that as medical advice is that you should be signed off sick completely, it would not be a good idea to allow you to keep working the hours you are doing.
She has given you the option of reducing your hours if you want to keep on working.
There is nothing wrong with her having a personal preference for you to keep working for a while. It would obviously make her life easier if you do, and there's nothing wrong with that. The important thing is she is not trying to make you work if you shouldn't be doing so. She is happy for you to be signed off in accordance with medical advice, and is offering this two day option for you to either take up or not, depending on what you want to do. You don't need to tell her where to 'stick it', you just need to decide what you want to do and let her know.
As I said in my post on the other thread, if your GP thinks he ought to sign you off sick altogether, I think you should allow him to do so, for your own benefit. Your health is the most important thing at the moment.
In terms of maternity pay if you do choose to reduce your hours and stay at work, statutory maternity pay is calculated based on average earnings during weeks 17-25 of pregnancy, so if you are earning a full salary during those weeks, your SMP will reflect that.
If you get more than SMP, you will need to check with your employer how that would work, and ask if any extra maternity pay could be based on your normal salary rather than the reduced one.