Boris Johnson's 10 Downing Street office said on Sunday that the Prime Minister and treasury chief Rishi Sunak were both alerted overnight by England's test-and-trace phone app. People who are notified through the app are supposed to self-isolate, though it is not a legal requirement. Contacts of positive cases usually are advised to self-isolate for 10 days.

from Times of India https://timesofindia.indiatimes.com/world/uk/in-u-turn-uks-boris-johnson-to-quarantine-after-covid-19-contact/articleshow/84522153.cms