The UK government advice has now been updated to cover additional areas. Full details are here but in summary.
If you have returned from these specific areas since February 19, you should call NHS111, stay indoors, and avoid contact with other people even if you do not have symptoms:
The specific lockdown areas in Northern Italy as designated by the Government of Italy
Special care zones in South Korea as designated by the Government of the Republic of South Korea
Hubei province, China (if you have returned in the past 14 days)
If you have returned from these areas since February 19th and develop symptoms, however mild, you should stay indoors at home and avoid contact with other people immediately and call NHS111. You do not need to follow this advice if you have no symptoms.
Northern Italy (defined by a line above but not including, Pisa, Florence and Rimini),
You can find specific guidance regarding self-isolation: