I don't think so. They can use a decentralized approach, the Google/Apple APIs, and still call it the NHS app. They can even put additional random opt-in featur

I don't think so. They can use a decentralized approach, the Google/Apple APIs, and still call it the NHS app. They can even put additional random opt-in features in their app if they really want to, why could they not? Plus it won't be practical at all to use a centralized solution on iPhone and it may be less practical to use it on Android, so this won't be just a slight reduction in usage, this risk to be a major one.