How is the Apple Watch ECG disabled in certain countries?
According to a few 2018 articles, the restrictions are based on the region where the watch was sold, and the removal of blocks is implemented by a watchOS update. 

Not tied to ID. No geo-fence. 

There was speculation that changing the watch's region settings could bypass the restriction, but those articles correct that understanding. 

Given these facts, the easiest way, and the way that fits the common programming design patterns, to restrict by region would be to disable globally, and with an OS update, enable the feature for the regionally purchased devices, likely by serial number/factory code.


As for which watchOS features are enabled for which country, Apple provides this handy list. The UK is listed as an approved country for ECG since watchOS 5.2 (March 27, 2019).