It's actually listed as an "ECG app coming later this year" on the Apple website, but I agree we don't know what that means exactly.We don’t know yet if it will be a stand-alone app that needs to be downloaded (and will be region locked to certain stores), or if it will be part of an OS update
If the ECG app is likely to increase the number of people wanting to seek medical attention then there's a decent chance it will never be accepted in the UK. The NHS is overworked, underfunded and too management/admin staff heavy to code with increases.
I don’t think that the UK/EU will withhold features because of resourcing restrictions. There are already ECG devices in these regions that have passed the regulatory testing. Apple just need European clearance and under that umbrella can launch the feature in the UK.