There are now 2 categories of response that returning travellers and/or visitors arriving in the UK should observe (the advice for Italy, Iran, and Daegu/Cheongdo applies to individuals who returned from these specific areas on or after 19th February 2020):

Category 1: Travellers should immediately self-isolate, even if asymptomatic, and call NHS 111 to inform of recent travel.
Affected Areas: Wuhan City/Hubei Province, Iran, Daegu or Cheongdo (Republic of Korea), any Italian town under containment measures.

Category 2: Travellers do not need to undertake any special measures, but if they develop symptoms they should self-isolate and call NHS 111 Affected.
Areas: Cambodia, China, Hong Kong, Italy, Italy (north), Japan, Laos, Macau, Malaysia, Myanmar, Republic of Korea, Singapore, Taiwan, Thailand, Vietnam.

For more information please follow the link below:
https://www.gov.uk/government/publications/covid-19-specified-countries-and-areas/covid-19-specified-countries-and-areas-with-implications-for-returning-travellers-or-visitors-arriving-in-the-uk