https://techcrunch.com/2020/05/05/nhs-covid-19-the-uks-coronavirus-contacts-tracing-app-explained/

The UK has this week started testing a coronavirus contacts tracing app which NHSX, a digital arm of the country’s National Health Service, has been planning and developing since early March. The test is taking place in the Isle of Wight, a 380km2 island off the south coast of England, with a population of around 140,000.

The NHS COVID-19 app uses Bluetooth Low Energy handshakes to register proximity events (aka ‘contacts’) between smartphone users, with factors such as the duration of the ‘contact event’ and the distance between the devices feeding an NHS clinical algorithm that’s being designed to estimate infection risk and trigger notifications if a user subsequently experiences COVID-19 symptoms.