I am in the same position. Born in the U.K. & are returning to live there indefinitely after 8 years here. Have continued to pay National Insurance contributions the whole time we have been here.
I will be 16 weeks pregnant when we return. I called a couple of GP surgeries & was told I would need to provide proof of permanent address, e.g utility bill before I could register. Didn't care that I was pregnant & need monitoring as developed pre eclampsia in my last pregnancy.
We have a holiday rental booked for a month when we return & will be looking for a long-term rental. I panicked a bit & emailed another surgery explaining my situation. I got a reply saying I can attend the surgery & use our temporary rental address in the short term, so we'll see how that goes....
In the meantime I have found a private scanning place & booked a scan for some peace of mind before I get into the NHS system.
I have had a look at the habitual residency test & it seems we need to prove we intend to remain there but it's a bit wooly on how to do that initially. Good luck!