I have the same issue, I was in the US 10 days ago (I live in the UK and I'm back home now), while I was there I decided to upgrade my Series 7 Apple Watch to the larger (46mm) Series 10 (did a trade in on my old one), and all was good, till I got back to the UK and decided to check my blood O2 and found it was no longer available.
At no point in the sale in the US did the Apple Store staff member say this would happen and I wouldn't have upgraded there if I'd known, I would have waited till I got home, and having visited an Apple Store in the UK now I've been advised that because I bought it in the US they are unable to do anything to help or exchange it.
There's no way to change the lockout on the watch as it's not one set by location (I have a UK based Apple Store account and there's no difference). The only way to get it back is for Apple to win their case and send out a Firmware update that unlocks it again.