I moved back to the UK (from the USA) at 6 months pregnant. I agree with most of what's been said here: you're eligible for NHS from the moment you arrive (because, as you've said, this is a permanent move home), and you'll need to register with a GP and then be referred to your local midwives team and whichever hospital you choose. (Or perhaps, if you're classified high risk, you'll be referred to consultant care?) You'll have a 'booking in' appointment and be issued with the standard NHS notes dossier. I brought all my American notes, but no one was particularly interested, and I ended up doing most of the preliminary blood tests again.
I found it really easy to register with a GP, once we were here just showed my UK passport and our rental contract but the hospital was more stringent about checking I was actually resuming residency, and not just popping back for some free maternity care. I was referred to their International Billing department, and had to bring in proof that we were back for good (in this case, DH's work contract).
I was wondering, though -- I know you said it's early days, but are you planning to have a Nuchal scan (NT test)? Is that included in the Hong Kong 12-week scan? (One thing I did find awkward, moving mid-pregnancy, was the similar-but-different schedules of care and testing; you get your head around one country's childbirth culture, and then suddenly things are different.)
The NT test has to be done during the 11wk-13wk+6 window. If you're moving at 12 weeks, then I'd make sure you had it done in Hong Kong, because it would be so stressful (and maybe impossible) to arrange it within less than a fortnight of arriving back in the UK.
Other than that, I think 12 weeks is a great time to move. 6 months wasn't actually too bad, but I did feel a lot of pressure to get our lives sorted out the moment we landed (all we had for the baby was a pair of socks and a Bruins hat). We moved to southwest London, too, which has a ridiculously high birthrate, but no problem with getting midwives and hospital spaces etc, so I wouldn't worry about that.