Hi
I work for the NHS and did through my pregnancy too. In my experience, I was allowed reduced hours (no overnight work, later start) and lots of flexibility, without the need to 'make up' the lost hours. My manager was reluctant to support me but once I had to be signed off for sciatica and SPD around 20 weeks for 1 week, she decided that she'd rather I worked flexibly than stopped working altogether which helped me carrying on working until 34 weeks.
What you get by law, in my memory, is paid time off to attend any appointment linked to ante natal care.
I think that I was told that as soon as you announce to your manager that you are pregnant, you have to do a risk assessment and agree if you are able to continue your duty safely, if not what are the options, adapt your current post, move to another post, not working etc.
If you are too unwell to work your full hours, it may be worth meeting occupational health to consider reduced hours if you are going to work at all. I don't think that they reduce your pay if you do that (they did not for me) but I am not a manager, your HR should give you a maternity policy about the local guidelines. The alternative might be for you to be signed off sick and not work at all and be on sick pay.
I think that sick leave has to be transformed into maternity leave if you are off sick by ??? 34 or 36 weeks of pregnancy, no matter when you planned to start it, your employer can force you to start maternity leave then.
Each trust has a family and pregnancy advisor, maybe they could help you with local policy? I found the occupational health team very supportive in my trust.
Hope you feel better soon, I had morning sickness all day long for about 6 months and used to stop to be sick regularly every morning during my work commute and the only thing that ever helped was to eat a cracker every hour or any form of bland carbohydrate that did not taste too strong :-)
good luck!