While the US is happy to allow the watch to use something Jobs' Mob calls an electrocardiogram (ECG) sensor, the UK is worried that it might be snake oil. 

The electrocardiogram (ECG) sensor measures the electrical activity of your heartbeat, providing you with a heart rhythm classification that can be shared with your doctor. It is expected tobe given the nod in the US because regulators are generally appointed by corporations to give the semblance of enforcement rather than actually telling a company no. 

According to 9to5Mac  reports,  it could take years for it to be approved in the United Kingdom because regulators tend to actually do their job. 

The Medicines and Healthcare products Regulatory Agency (MHRA) explains that the process starts by examining Apple's documentation surrounding the ECG feature and performing an audit of the quality assurance system. While this step doesn't appear to be lengthy, the proceeding steps could make the process longer. MHRA says it would require Apple to perform a new clinical investigation to judge the effectiveness of the ECG on Apple Watch, but Apple would likely not be able to use any of the data from the studies it's already completed because MHRA requires companies to notify the regulator in advance of a study.

Once the study is submitted, MHRA has 60 days to approve it (which may become longer if the regulators have further inquiries for Apple), and then Apple can begin the study. These last few steps are what the MHRA say "could potentially add years" onto the debut of the ECG in the UK.

While the United Kingdom remains part of the European Union, it's possible that Apple could receive approval from a broader regulatory body and sidestep the MHRA's processes.