4Mrs, I think that you can "push forcefully" for what you want. Unless there is a specific medical risk which means it would be inadvisable for you to have a particular type of birth, you should be able to get what you want.
I know that there is a lot of info online about getting home births when midwives etc try to refuse (I did some research for my SIL at the point where she was threatening to just give birth at home on her own if they refused a homebirth based purely on no of children). Lots of the advice would probably be useful to argue for a water birth.
If you get details of why you are considered high risk and have done plenty of research around your preferred birth then talking to the head midwife should sort things out.
Even if there's a compromise to be reached, eg your risk is because of pph in all previous births so you can labour in a birthing pool but have to actually give birth on dry land in the hospital.
I think that if you can show that you're aware of the risks then you tend to be taken seriously.
Maybe raise it with your consultant and see what they say, and if it's a no then ask for detailed reasons then go away and research. You have 20 odd weeks to research!
Stacey, it's taken as 24 weeks, which is why you still get full maternity benefits/ leave etc even for a still birth after that point. But a colleague's son was born at about 22 weeks and is home but with lung issues.