Yes, I'm fairly sure you can buy AppleCare in Japan with the initial device purchase as a non-resident. https://www.apple.com/jp/shop/buy-wa...E3%83%89-sm_ml lets me add a mock order with AppleCare to the cart and I was able to purchase the JP watch series 2 online (via Apple Friends & Family) as a tourist a few years ago. No tax-free for online orders, though.

Varying reports on if AppleCare+ is truly global https://www.reddit.com/r/apple/comme...reign_country/ I was under the impression that it would be valid between the US and 2nd country if sells that sku or has like for like stock to replace it. I could be misinformed.

The UK may have different consumer protection laws
https://support.apple.com/en-us/HT202696
https://www.apple.com/legal/warranty...oryrights.html

/OT
If you go to Japan, Suica Eng(< iOS 13) on the iPhone/Watch is pretty convenient for reloading suica credit via foreign credit card https://atadistance.net/2019/03/24/t...a-english-app/ or iOS 13 Wallet https://atadistance.net/apple-pay-suica/. Physical suica cards are generally loaded by cash (for tourists) or domestic CC. It would work with an Apple Watch Series 3. Apple Pay & Suica on iPhone/Apple Watch in Japan [consolidated thread] Suica can also be used in Osaka/Kyoto where Icoca is accepted and instead of Tokyo Metro Pasmo except for day tickets and commuter (monthly) tickets.