Atrial Fibrillation is believed to be greatly under-diagnosed given its sporadic in nature, especially during early to middle age. Even in older people, like myself, it's very hard to capture and diagnose given the occurrence rate is too low to be reliably picked up by routine ECG tests. People experiencing mild atrial fibrillation often are unaware of it.
Yet this disorder has a high correlation to strokes later in life. Your first indication of a problem could be a disabling stroke.
The beauty of the Watch Series 4 (when the software arrives) is that it potentially can monitor you 24x7, and as a result will eventually detect when AF occurs, even if it's very sporadic or related to certain activities. (Assume for the moment that sex is what trigger's your AF... kind of hard to capture that moment in the doctor's office. Or only when you're hiking up a steep hill. Or having a strong emotional reaction to something.)
The challenge then is to get your cardiologist on board. The problem is exponentially worse if you don't already have a cardiologist that you see. I'm not aware of a standard stress test that can be used to reliably reveal the problem.
Some docs will embrace the opportunity for 24x7 monitoring, and others will be less receptive. That might depend on how they get paid (in the US anyway).
For my part, as soon as the software is available, I'm going to be talking to my cardiologist about how to get him plugged in. Assuming he's not already welcoming this feature.
The difference in the US versus the UK is that docs in the US have significantly more freedom in how they respond to new technology, given that healthcare in the US is mostly a private business and docs can set their own rules (within reason).
That said, it will be interesting to see how private insurance handles compensation for doctor visits triggered by the Watch. Preventing a stroke by detecting AF and treating it early (with meds) is a LOT cheaper than treating the patient after a stroke, so likely they'll view this as a good investment. Insurance in a free market environment is always looking at the lowest total cost of treatment.